Sing to me, O Muse, of BECMI!

[deganawida]

With the ongoing kerfuffle, I am reviewing things that I want to run with my friends and family. I'm just a backup, one-off DM for our group, typically, but I want to play less and help others play more. As part of that, and noting that I'm putting together a Savage Worlds short campaign and a Castles & Crusades version of the DL modules, I thought to look at something which I never played but always wanted to play in or run: BECMI D&D. I have OSE, and love what it has of B/X, having never played B/X when it was big, but BECMI seems to be a total gaming system, and the Known World/Mystara enthralled me in my teens with Tower of Doom and its sequel.

Things which appeal to me is that it seems to be a complete game, with a world and lore baked in, and that the story should emerge during play. By the latter, I mean that it seems (from my brief review of web reviews) to start in a dungeon or wilderness, and player action starts to form antagonists as they go. I may be drawing conclusions there, but that concept of play really intrigues me.

So, for those of you who have experience, what separates BECMI from AD&D, both in rules and flavor? What is good about it, what is not good (Thief Skills, I know about), and how should a campaign for this be run? Would it be best starting with the B and E box sets? Or should I just grab a copy of Rules Cyclopedia off of eBay and make something from that?

Also, what do y'all think of the Known World/Mystara as a setting?

 

[Ralif Redhammer]

I have a soft spot for BECMI and always will. By my book, the basic set is the most concise and appealing presentation of the D&D system, giving you all you need to get started and drawing you into what makes the game so special.

Definitely starting with the B and E parts. As for how it's run, the BECMI progression kinda goes like this: dungeons (B) > hexcrawls (E) > tourneys/armies/kingdoms (C) > rulership & potentially questing for godhood (M) > godhood (I).

 

[DarkCrisis]

I love the Rules Cyclopedia. The best one and done D&D book you can buy.

 

[Yora]

I think the Basic and Expert rules are actually the best RPG ever written. And that's from someone who didn't really know what they are until 6 years ago.
There's plenty of weird quirks that could have been a lot better, but it was still in the first decade of RPGs being a thing and it had to follow the footsteps of OD&D, so that's understandable.

But what the system does really well, which I think I've not really seen anywhere else, is that it's laid out and structured as a game. A game with clear procedures, a goal, and an objective measure of success and progress. Search for a dungeon, explore the dungeon, collect as much treasure as you can without getting yourself killed, return with the treasure to a town. And then count the points you made. That's a simple procedure that players can follow when they know nothing about the world and have no ideas what kinds of plans would be possible or interesting for their characters to pursue. And most importantly, it's something that the players can do on their own, without the GM telling them where they should go and what they are supposed to do when they get there.

Roleplaying is something that just starts to develop naturally during the course of playing the treasure collecting game, and in the process the interactions of the players with the game world introduce them to things that are going on and to other people who play important roles in that world. And that opens the door to PCs pursuing other goals alongside the treasure collecting.

The Basic rules give the players something to do without any need for guidance from the GM whenever they don't know what else their characters should or could do at that specific moment. Just ask around for any leads about unexplored ruins in the area. Adventure will follow from that. And even with me repeating myself because it's that important, without the GM telling the players where the adventure is and what will happen in it.

I think the Dragonlance innovation of having the players experience a prepared story was the biggest mistake in the evolution of RPGs. The Basic, Expert, and Companion rules are a lot smaller in scope of what they offer to the players, but the thing they do offer is a prime setup to have something that develops into something much greater than what most other RPGs can provide.

 

[Haplo781]

BECMI is the 2nd best D&D after 4e and Myatara is the 2nd beat setting after Nentir Vale

 

[ilgatto]

I think the Basic and Expert rules are actually the best RPG ever written. And that's from someone who didn't really know what they are until 6 years ago.
There's plenty of weird quirks that could have been a lot better, but it was still in the first decade of RPGs being a thing and it had to follow the footsteps of OD&D, so that's understandable.

But what the system does really well, which I think I've not really seen anywhere else, is that it's laid out and structured as a game. A game with clear procedures, a goal, and an objective measure of success and progress. Search for a dungeon, explore the dungeon, collect as much treasure as you can without getting yourself killed, return with the treasure to a town. And then count the points you made. That's a simple procedure that players can follow when they know nothing about the world and have no ideas what kinds of plans would be possible or interesting for their characters to pursue. And most importantly, it's something that the players can do on their own, without the GM telling them where they should go and what they are supposed to do when they get there.

Roleplaying is something that just starts to develop naturally during the course of playing the treasure collecting game, and in the process the interactions of the players with the game world introduce them to things that are going on and to other people who play important roles in that world. And that opens the door to PCs pursuing other goals alongside the treasure collecting.

The Basic rules give the players something to do without any need for guidance from the GM whenever they don't know what else their characters should or could do at that specific moment. Just ask around for any leads about unexplored ruins in the area. Adventure will follow from that. And even with me repeating myself because it's that important, without the GM telling the players where the adventure is and what will happen in it.

Although I'm not a huge fan of BECMI for a number of reasons (and not especially opposed to story-telling adventure modules) I think you make some very good points here. In fact, I agree with your observations on how people should learn to play the game so much, that I had to react to your post as I presently do.

 

[rogueattorney]

With the ongoing kerfuffle, I am reviewing things that I want to run with my friends and family. I'm just a backup, one-off DM for our group, typically, but I want to play less and help others play more. As part of that, and noting that I'm putting together a Savage Worlds short campaign and a Castles & Crusades version of the DL modules, I thought to look at something which I never played but always wanted to play in or run: BECMI D&D. I have OSE, and love what it has of B/X, having never played B/X when it was big, but BECMI seems to be a total gaming system, and the Known World/Mystara enthralled me in my teens with Tower of Doom and its sequel.

Things which appeal to me is that it seems to be a complete game, with a world and lore baked in, and that the story should emerge during play. By the latter, I mean that it seems (from my brief review of web reviews) to start in a dungeon or wilderness, and player action starts to form antagonists as they go. I may be drawing conclusions there, but that concept of play really intrigues me.

So, for those of you who have experience, what separates BECMI from AD&D, both in rules and flavor? What is good about it, what is not good (Thief Skills, I know about), and how should a campaign for this be run? Would it be best starting with the B and E box sets? Or should I just grab a copy of Rules Cyclopedia off of eBay and make something from that?

Also, what do y'all think of the Known World/Mystara as a setting?

You’re kind of asking about three separate but related things here - the BECMI rules, the Rules Cyclopedia and the Known World/Mystara setting. There are a ton of products that fall under these headings that came out over a decade of time.

So, I’ll address them one at a time.

1. BECMI rules, specifically the 5 boxed rules sets that came out from 1983-86. These were all edited by Frank Mentzer and written by a whole host of different contributors. The Basic and Expert sets give you rules that are indistinguishable from either the prior 1981 B/X sets or the recent OSE baseline, are largely compatible with 1970s OD&D products, and are extremely similar to Labyrinth Lord, BFRPG and several other clones. There’s a very little bit of info on the Known World setting, specifically Karameikos. Far more info on the Known World is given in the module X1 Isle of Dreas, which was included in the original 1983 boxed set, but isn’t, i don’t believe, included with the pdf on DTRPG.

I love these rules, but the 1983 set is not my favorite delivery of these rules to an already established player. It’s set up as a tutorial. It’s fantastic for teaching someone brand new to the game, but not as great as a reference for old hats who just want to quickly look up a rule. The earlier 1981 Basic set is better for that.

The Companion and Master sets are a mixed bag of rules add ons. Some of it is fun to tinker with - the dominion play and mass combat systems for example. Some of it is inspiration and encourages you to take your game in strange new directions - the Demi-human relics and quests for immortality. Some of it is a clunky rules mess - in particularly weapons mastery.

There are several options presented as for high level play for which there really isn’t any reason for them to restricted to such high levels of play - the Paladin, druid and avenger proto-prestige classes, for example. And the focus on Uber-high level play in general is something that will simply never come into use for a very lot of groups.

Like the B and E books, there are a lot of hints and blurbs and setting assumptions that give you a look into what the Known World is like, but it’s really skeletal.

Personally, I think these are best used as a grab bag of ideas to be picked through and not implemented whole hog.

The Immortals boxed set is essentially an entirely new game for world hopping Demi-gods. This box more than any of the others was exclusively Frank Mentzer’s baby, and in a lot of ways it’s far more idiosyncratic than the others. There are a lot of ideas in it about the implied cosmology of the D&D game world, but I’m not sure I’d ever be in a place to run it as is.

2. Rules Cyclopedia - The RC is a 1991 compilation of most - but not all - of the rules from the B, E, C and M boxes. It has nothing from the Immortals rules. It adds some rules from the GAZ series of supplements, mainly a sampling of non-combat skills and different magical research rules than from the E box. It had a few maps and a brief gloss over the Known World setting.

I find the RC a mixed bag. It has a lot of stuff I would never use, it’s something of an editing mess - most infamously the combat section, it does not contextualize the rules from several different sources and authors into anything like a cohesive whole. I wouldn’t recommend anyone try to learn to play the game from this book. But as a one stop shopping single-volume, everything you could possibly want for a campaign in a single book product, it’s about as good as official D&D has ever gotten.

3, Known World/Mystara - one of the weird things about the setting is that there was never a single campaign world-wide source to summarize the setting. There were several distinct phases of product for this campaign world. In the context of BECMI, it was first detailed through the adventure modules that were produced contemporaneously with the rules boxes from 1983 to 1987.

It gave you a jigsaw puzzle of not-particularly well connected empires, lost civilizations, inhuman races, other dimensions, other planets, and other timelines for characters to bumble through in an 80s Saturday morning cartoon sort of way. It’s a gloriously incoherent fantastic mess of a disaster.

Many of the adventures in the B, X, CM, M, and DA series are not particularly good for the adventures themselves, but have some great blurbs of setting content.

In 1987, TSR started working to integrate their Known World into something more cohesive and started the GAZ series of supplements, each focusing on an individual kingdom in the Known World. These were done by several different authors in several different authorial voices, mostly free lance and it’s a veritable who’s who of future prominent 90s rpg designers.

Many of the GAZs are very good. Some, particularly towards the end of the series are awful. Many have something of a flippant tone, which turns some people off. They are also all presented as from the perspective of the inhabitants of the kingdom being covered. So there are often contradictions from GAZ to GAZ. Also, several aspects of the world as presented in the older adventures and rules books were completely changed in the GAZ era books.

Later there were several boxed sets expanding the campaign world. Dawn of Empires dealt with two big empires to the East. Hollow World dealt with a lost world type setting below the Known World’s crust. Champions of Mystara dealt with some of the nations to the West.

The cartography in the GAZ series is universally excellent. If you like hex crawling; the maps alone might be worth it. Also, later on there were the PC series dealing with non-human character classes and HWR series dealing with nations in the Hollow World beneath the Known World. Both of those series are of similar depth and quality to the GAZ series.

 

[Ralif Redhammer]

In my experience, role-playing started right in the intro adventure in the Red Box, the moment I started trying to find a path that would save Aleena. The moment I started imagining myself as that plain old Fighter.

Roleplaying is something that just starts to develop naturally during the course of playing the treasure collecting game, and in the process the interactions of the players with the game world introduce them to things that are going on and to other people who play important roles in that world. And that opens the door to PCs pursuing other goals alongside the treasure collecting.

Dragonlance the setting, I love. Dragonlance the modules, not so much. I've been idly re-reading them and the first one is actually pretty good and has more sandbox to it than I remembered, but as the series continues, where they run into trouble is when they tries to precisely curate the player experience and story. It grips onto the plot so tightly that it prevents the adventure from unfolding naturally, from generating those spontaneous moments of genius.

I think the Dragonlance innovation of having the players experience a prepared story was the biggest mistake in the evolution of RPGs. The Basic, Expert, and Companion rules are a lot smaller in scope of what they offer to the players, but the thing they do offer is a prime setup to have something that develops into something much greater than what most other RPGs can provide.

 

[Yora]

In my experience, role-playing started right in the intro adventure in the Red Box, the moment I started trying to find a path that would save Aleena. The moment I started imagining myself as that plain old Fighter.

Yeah, but isn't all of that a straight up case of pre-written, scripted experience?

Not saying it's not introducing roleplaying, but it's not quite an example of what I was talking about.

 

[Ralif Redhammer]

It is indeed, with a scant handful of decision points and dice rolls.

Yeah, but isn't all of that a straight up case of pre-written, scripted experience?

Not saying it's not introducing roleplaying, but it's not quite an example of what I was talking about.

 

[CleverNickName]

Would it be best starting with the B and E box sets? Or should I just grab a copy of Rules Cyclopedia off of eBay and make something from that?

You could do that, sure. My advice would be to just pick up a print-on-demand hardcopy of the Rules Cyclopedia from DriveThruRPG for $25...probably cheaper than what you'll find on eBay. And while you're there, you could grab a copy of GAZ1, The Grand Duchy of Karameikos for another $10 -- it's a really good "deep dive" into the default starting point for most of the BECM adventure modules.

 

[rogueattorney]

You could do that, sure. My advice would be to just pick up a print-on-demand hardcopy of the Rules Cyclopedia from DriveThruRPG for $25...probably cheaper than what you'll find on eBay. And while you're there, you could grab a copy of GAZ1, The Grand Duchy of Karameikos for another $10 -- it's a really good "deep dive" into the default starting point for most of the BECM adventure modules.

I certainly recommend getting the RC off of DTRPG instead of ebay. The ebay prices are astronomical.

That said, I'd reiterate that the RC is a strangely edited book with several odd additions, omissions and alterations that do not seem to have been play-tested and may not have even been intentional. Anyone who does not have a background in the source material for the compilation is going to find a ton of weirdness that isn't going to make much sense.

Just one example of dozens - the section on Thief skills notes that a failed "remove traps" roll automatically sets off the trap. This wasn't the rule in B/X or BECM, nor was it the rule in either 1e or 2e AD&D. I have no idea where it comes from.

It's a bad rule considering the thief's really bad chance of succeeding. 10% at first level, still only 38% at 7th. In the prior editions, you give it a shot since there weren't any bad consequences for failing. Using the RC, the thief almost never exposes himself to the harm given the severe consequences for failure.

As I said, there are several other instances like this with changes that may have been incidental during the compilation process that gives strange results. Most infamously is the movement during combat section that seems limit movement in combat to 5' per round.

 

[Snarf Zagyg]

A few thoughts-

1. Awesome! Also ... YA BASIC! Heh. Seriously, though, lots of fun to play.

2. B/X is not identical to BECMI. BECMI is not identical to RC. They are all largely alike, but not the same. (And Holmes Basic is ... just not at all the same- it's OD&D simplified).

With that in mind, I am going to advocate for Moldvay/Cook (B/X). It is, in my opinion, the simplest and best D&D ruleset out there. If nothing else, what happened to the Thief in Mentzer was a crime.

 

[Ralif Redhammer]

I grew up with BECMI, but I picked up Moldvay Basic last year and it really is an impressive work in how concise and well-written it is.

A few thoughts-

1. Awesome! Also ... YA BASIC! Heh. Seriously, though, lots of fun to play.

2. B/X is not identical to BECMI. BECMI is not identical to RC. They are all largely alike, but not the same. (And Holmes Basic is ... just not at all the same- it's OD&D simplified).

With that in mind, I am going to advocate for Moldvay/Cook (B/X). It is, in my opinion, the simplest and best D&D ruleset out there. If nothing else, what happened to the Thief in Mentzer was a crime.

 

[Velderan]

A few thoughts-

1. Awesome! Also ... YA BASIC! Heh. Seriously, though, lots of fun to play.

2. B/X is not identical to BECMI. BECMI is not identical to RC. They are all largely alike, but not the same. (And Holmes Basic is ... just not at all the same- it's OD&D simplified).

With that in mind, I am going to advocate for Moldvay/Cook (B/X). It is, in my opinion, the simplest and best D&D ruleset out there. If nothing else, what happened to the Thief in Mentzer was a crime.

What are the major differences between BECMI and B/X? I keep hearing thief skills were pretty poorly thought out in BECMI (I think, I may have that backwards..) but aside from that difference why B/X?

 

[CleverNickName]

What are the major differences between BECMI and B/X? I keep hearing thief skills were pretty poorly thought out in BECMI (I think, I may have that backwards..) but aside from that difference why B/X?

There are no major differences, only minor ones. You can play the two interchangeably (ask me how I know!) with just a bit of occasional handwaving.

 

[Snarf Zagyg]

What are the major differences between BECMI and B/X? I keep hearing thief skills were pretty poorly thought out in BECMI (I think, I may have that backwards..) but aside from that difference why B/X?

Levels. The Thief was weak in B/X. In order to stretch out the levels, Mentzer made the Thief unplayable by reducing the ability to succeed at ... you know ... thief skills.

It was also reflected in, inter alia, saving throws and spell progression. B/X plays must better at all the levels you're likely to play.

 

[Willie the Duck]

The BECMI rules are really good where they are really good, and when they are bad tend to fall into the categories of missed opportunities, kludgey, or the same things that all D&D/AD&D at the time did. More discussion about the good at the end.

The first issue is the kludge -- while it is clear (from what we know of the company history, and just that they already had a basic-expert version of the game with Moldvay-Cook) that TSR intended to produce some or all of C, M, and I from the beginning, but from the actual products sometimes it is hard to tell that. Demihumans cap out at the same levels as in B/X, but the game keeps going up in level (upon which an unnecessarily awkward form of non-levelled-advancing is grafted on to them). Weapons like tridents are introduced in the Companion set. Weapon Mastery and some other combat options are included in the Master Set. Domain rules are in the Companion Set despite rulership starting at name level. Most notably the first printing of the B and E books used the same thief ability progression as Moldvay-Cook BX, but then subsequent printings changed them to fit a trajectory where thief abilities could approach 100% at 36th level*
*My personal experience was first playing with BX, getting first printing BE as the first books I owned, and then collecting C, M, and I as they came out. We were all mightily confused at what exactly was going on.

Second is things all of D&D had in the TSR era. These pop up in discussion about BX and AD&D as well. These are the things that people sometimes dislike the implementation of the replacement in WotC-era D&D, but probably understand why the change was made. Such as: I don't think level limits to balance demihuman abilities* ever worked well, but if you are going to do so, placing the hardest cap on halflings and the least stringent on elves never made sense. Everyone loved thieves and roguish characters as a concept, but no one ever wanted to play them as-is** because they were barely less fragile than magic users in exchange for the grand luxury of... some dungeoneering skills, many of which subject you to peril if you fail, and which other classes may or may not be able to replicate with spells or actions-anyone-can-take. Lots of 'be perfectly prepared for anything at all times' deathtrap monsters. And so on.
*or to make it a humanocentric game, as the purported real reason was
**Everyone I knew house-ruled something. I think we gave them a fighter chasis, and just traded being able to wear chain and plate for thief abilities.

And then there are missed opportunity. Firstly, Ralif is right that "the progression kinda goes like this: dungeons (B) > hexcrawls (E) > tourneys/armies/kingdoms (C) > rulership & potentially questing for godhood (M) > godhood (I)". Second, Yora is spot on that the game's strength is "A game with clear procedures, a goal, and an objective measure of success and progress. Search for a dungeon, explore the dungeon, collect as much treasure as you can without getting yourself killed, return with the treasure to a town." The missed opportunity, I feel, is in not fleshing out and making enjoyable the systematic components of the rest of what Ralif mentions not encompassed by what Yora mentions. The game does a spot-on job of setting up mechanisms for a procedurally rigorous and challenging (and enjoyable, if you enjoy the specific challenges upon which it focuses) dungeon-crawling experience. And then the expert set sets up the procedural wilderness mechanics that... do a really good job of determining how long it takes and how many encounters you go through to get to the next dungeon you can go back to procedurally dungeon-crawling. It doesn't really do a great job of making the wilderness hexcrawling interesting or fun, but more an obstacle or resource drain you seek to minimize so you can hit the next dungeon at full HP/spells/arrows. I bet you (whomever's reading) had a great time wandering the wilderness in BECMI back when you played the game BitD, and I also bet a good and imaginative DM was involved. I think that's why so much OSR ink is spilled on hexcrawl creation -- recapturing and proceduralizing what everyone made houserules/solid DM fiat to do. The Companion set rules for tourneys/armies/kingdoms is also a set of procedural rules for which I think a lot of house rules and DM inventiveness supplemented. They're not bad, they just, seem like they're there because someone thought they had to be (in case someone asked 'So what do I do with this keep and followers? How much revenue do I bring in? What happens if I declare war on my neighbor?'), more than built to be actually fun to play for extended periods. The Master Set quest for godhood, on the other hand, is really exciting and engaging and I remember plenty of people getting really excited about that and getting back into their high-level characters when it came out. In this case, things have flipped -- it seems fun as it's own thing and not just support structure for more dungeon-crawling (or planar adventure, as high-level often switches over to) and the like, but in this case the procedural structures are rather thin and it becomes very freeform/DM adjudication (again, to which I think lots of people had fun doing, because they had DMs who could do it well). The Immortal set -- I gotta be honest, I remember loving questing for immortal status, and loving the bizarro reality of the BECMI planes and laws governing the universe (which seemed so much more imaginative than AD&D's wheel of alignment-planes), but no one I knew really spent much time playing as immortals. I think there was some confusion about what exactly you were supposed to do or even want at that power scale.

After all those caveats, I do think that BECMI is a good game. If for no other reason than because it does well the part that I think most people actually want out of their D&D-like games rules structures. Sure, I would love a hugely rigorous and fun for its' own sake wilderness adventure rules and I'll bet someone else wants a great domain management game, but most people (I think) want a game where you go through dangerous environments and use your awareness, cleverness, and appropriate use of limited resources to evade traps, defeat nasties, and acquire treasure. Compared to WotC-era D&D, it has the (arguable, I know) advantage of keeping you hungry and scared. Your expendable resources are pretty scarce, you're fairly vulnerable to a fair fight or un-found trap, and a lot of your power coming from last adventure's loot. That makes mere survival or some potions or a +1 spear you don't need but your henchman can sure use still mean something six or eight dungeons into your career. The biggest advantage, I feel, is gp=xp -- although I think some OSR types overplay the value, and you definitely want to modify something if you aren't going to play dungeon-crawler--in that it gives a solid explanation for why you are down in these dungeons where the game rules want you to be and also solidly defines success (having acquired treasure, and lived to take it back to town).

Definitely starting with the B and E parts. As for how it's run, the BECMI progression kinda goes like this: dungeons (B) > hexcrawls (E) > tourneys/armies/kingdoms (C) > rulership & potentially questing for godhood (M) > godhood (I).

But what the system does really well, which I think I've not really seen anywhere else, is that it's laid out and structured as a game. A game with clear procedures, a goal, and an objective measure of success and progress. Search for a dungeon, explore the dungeon, collect as much treasure as you can without getting yourself killed, return with the treasure to a town. And then count the points you made. That's a simple procedure that players can follow when they know nothing about the world and have no ideas what kinds of plans would be possible or interesting for their characters to pursue. And most importantly, it's something that the players can do on their own, without the GM telling them where they should go and what they are supposed to do when they get there.

Personally, I think these are best used as a grab bag of ideas to be picked through and not implemented whole hog.

I think that's almost necessary. There's a lot of stuff that either isn't necessary, isn't everyone's cup of tea, doesn't work in a given context, or just don't come together to something meaningful unless it's a central game focus.

With that in mind, I am going to advocate for Moldvay/Cook (B/X). It is, in my opinion, the simplest and best D&D ruleset out there. If nothing else, what happened to the Thief in Mentzer was a crime.

I don't disagree. That said, the B/X thief was also... an interesting design choice to which I do not agree. Honestly, pretty much all TSR thieves (mechanically-speaking. Obviously the character concept of the thief is pretty engaging) seem like also-rans. From their introduction in oD&D Supplement I, the thief was set up as giving up just about everything (decent HP, AC, saves, attack value) in exchange for their abilities (and a more favorable xp chart which usually only has them a level or two ahead of others). Exactly why was never clear to me, and I sure think that the game would have worked very well with a thief class which was kind of a mirror of the cleric -- fighter-like attack instead of fighter-like defense, d6 hd, reasonable saves, and some late-game abilities (like cleric's higher level spells) the accounted for the games' shifting focus. I know in all versions of TSR-era A/D&D (including both BX and BECMI) we houseruled the heck out of thieves unless you played multiple characters (in which case if the thief was a swiss army knife who did nothing until a trap check was needed, it didn't matter as much).
*in retrospect I think Gygax may have preferred the 'anyone can climb and hide, and you find traps by saying you are searching the right spot the right way' method of pre-Greyhawk, and just included thieves for anyone who didn't.

 

[Willie the Duck]

What are the major differences between BECMI and B/X? I keep hearing thief skills were pretty poorly thought out in BECMI (I think, I may have that backwards..) but aside from that difference why B/X?

At the levels where they both play, and if you do not use rules introduced in post-expert BECMI, I think the main rules differences are the thief skills and that BX Magic Users had more limits on how many spells they could learn (IIIRC a BX MU has the same number of spells per level as their cast-per-day limit for that level, and can only up that with post-name-level spell-research; BECMI has more nuanced rules which allows some more options to find and add more spells to their spellbook). Other than that, it is mostly presentation. BX made the procedural aspects of dungeon-crawling front and center. BECMI included them, but didn't make them clear*, nor explain why you should want to follow them when the DM could just wing it. A lot of 8-10 year olds picking up BECMI missed vital parts of the exploration procedure.
*I am saying this as someone whose first or second fictional-character-crush was on Aleena (after Eilonwy from the Chronicles of Prydain books) -- the BECMI "choose-your-own-adventure"-style introduction did a disservice in that it showcased the character-sheet component of the game, the combat and spell resolution component, but completely omitted the dungeon-crawling procedures, morale checks, the reaction table, and other vitally important game mechanisms that countless of us didn't pick up on the first time through the game.

 

[Voadam]

What are the major differences between BECMI and B/X? I keep hearing thief skills were pretty poorly thought out in BECMI (I think, I may have that backwards..) but aside from that difference why B/X?

B/X are the 1981 Moldvay Basic and Cook Expert sets, covering levels 1-3 and 4-14. It said the game would go on to level 36 and it provided the hints of Mystara in the expert set.

BECMI are from five Mentzer sets (Basic, Expert, Companion, Master, Immortals) starting in 1983 covering 1-36 and immortals after 36.

So BE of BECMI is basically B/X. One difference is thief skills do not advance as much in BECMI so that they improve through 36 instead of getting more powerful high level thief abilities as hinted at in the B/X expert set.

Companion set added in domain running rules, some higher level demihuman stuff for after they stopped advancing, and some class stuff that worked like subclasses that could be unlocked at higher levels. Master set added in the really cool weapon mastery rules which continued in Rules Cyclopedia (basically cool weapon feats allowing things like parrying with a sword that really bumped up fighters). Immortal set was a whole thing of itself.

The basic set in BECMI is also set up differently from B/X. B/X Basic is one compact rule book with great advice on playing D&D. BECMI is set up as a player's and a DM's book with a choose your own type adventure to get a player going.