They mentioned being more cautious in their approach to releasing material. It seemed as though the idea for D&D Durango to have a slower release schedule than 3rd or 4th did.
While I applaud that in the sense that many simple mistakes which made it through to final products in previous editions might be caught now that more time is (supposedly) between products, I am cautious about what that means for the modular approach. If the core kernel of the game is not capable of popping into the experience I want right out of the gate, there might be a long time until I get the tools I need to view D&D Durango as something I want to spend money on. Likewise, since I often feel as though my interests place me in the minority when it comes to what D&D players enjoy, I'm not sure what to expect as far as how long I'd have to wait for D&D to be a game which can tell some of the stories I want to tell.
They do point out in the video, the initial book will not just have the core, but a lot of modular elements right out of the gate. [Adding tactical grid combat, probably some variations on critical hits and fumbles, different rules for dropping below 0 hp, stuff involving death by massive damage and coup de grace perhaps, etc]. Now, it's possible stuff you are looking for aren't in the initial wave, but if you have some idea what the modular element would be, it is probably easy to add your own house rules. The game, in effect, is a small core system with a bunch of suggested house rules included.
I think slowing down product release is good for a lot of reasons. I am somewhat cautious how that will mix with the modular approach though. How much material the core game covers will be of interest too.
From the sounds of it, a lot of the modular elements are things that shouldn't effect adventure design too much. While there will hopefully be maps that allow you to use the grid for combat, you'd still want the maps to give you an idea of the layout for gridless combat. You don't really need to change the numbers if you are changing what happens when you reach 0 hp, or what happens when you roll a 1 on your dice. Similarly, character creation isn't significantly changed by most of the DMs modular options (it would seem). While you may make different choices knowing whether there will be no, little, lots or all grid combat, there really isn't an element on the character sheet that would need to be modified to reflect that. And, from the sounds of some of the rumors anyway, the player options is mostly core stuff. You can go with a theme, or you can customize your feat selection. You can go with a background, or you can customize your skill/language selection. Based on skill selection, your fighter may just have a bunch of attacks like in 3.5, or you can have a number of combat maneuvers like 4e.
The player designs their character however they want, and the DM creates the environment the players are dropped into. The player/monster interaction ultimately comes down to the core rules of d20's modified by ability scores and other modifiers. Most of the modules likely won't modify those greatly, or at least, won't require different character sheets (or adventures) for each module. Instead you just apply the effects of the module to the edges. Rarely does an adventure get into the marginal elements that most modular things will likely cover. I don't know many adventures that address what happens if you drop below zero hp, or if you roll a critical failure. Now, you may have to adjust the ammount of treasure based on magic item rarity, but magic items in a setting is often one of the most likely to be changed anyway. [And, there may even be a modular approach to giving out treasure, so that the adventure just has to point out where treasure goes, and the DM decides which method to use to determine the treasure there ... instead of the adventure doing so for the DM]. Treasure allocation is, again, a modular element which doesn't actually change monster design or PC creation, etc.
Finally, there is the issue of how DDi interacts with the modular approach. If some of the things I enjoy (which, again, I would say place me in somewhat of a minority) to fill a whole book, and there is only enough for a Dragon or Dungeon issue, I'm not a subscriber, and won't have those options available. Other gaming companies handle this issue by using pdfs; for example, when I bought the book of GURPS Low-Tech, there were also optional 'companion' pdfs which I bought to add to the material found in that book. How I got those pdfs was simply by buying the pdfs; no subscriptions to services or anything of that nature was necessary.
It does depend. Most modular elements will be combined and included 'in bulk'. So, it would be more about where they fit thematically. So, if they put out a Dark Sun book, it would include modular elements for dealing with desert environments, non-metal weaponry, low magic item setting, wild talents, etc ... things iconic of that setting (assuming they didn't already appear elsewhere; and depending on the ammount of page space, they could just reprint it in the book anyway). Now, it's quite possible that what you are looking for is a few things that don't have a home in a specific book, but instead work in an article, in which case, outside of a "best of Dragon" release, would only be available through the website, you may be out of luck [or need to do a one-month subscription, grab some back issues thing once in a while].
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As for the idea of 'modern rules/old school feel', one of the things that was common, especially for 2e, was the kitbashing, houseruling elements, where no two DMs were really doing the same thing. While the lack of rules, and thus freeform play was an element back then, for many at least, it wasn't a constant game of DM making stuff up, it was often a case of DM makes stuff up, but going forward, that's the rule. Either the DM thought out the house rules ahead of time, or at least tried to be consistent once they made something on the fly, etc. So the 'modern' thing is likely to do some of the DMs work ahead of time. "Everyone is going to running a house ruled version of D&D, and here are a list of possible house rules we've come up with to help you get started." So, for people used to the modern style of the rules of the game being in print, they go out of their way to show all these rules that are optional, instead of mandatory. The hope is that, it gives the DM the freedom to go further, and not just use the optional elements in the book they like, but that in seeing the areas where the rules are amorphous, they can make their own changes without it feeling like 'cheating' or being a 'bad DM' like it might to houserule a more rigid system like 3.5 or 4e.
Obviously, there is the risk that giving the DMs a list of choices for houseruling elements will blind them to anything outside that system, just as some people find that a character with all the tactical options a 4e character does will rarely think outside the box to try a "page 42" type maneuver or something similar. However, it's more often a complaint of the DM [or other players] that someone fails to see innovative options instead of sticking to the power cards, while it's less likely to be a situation where the players are upset that a DM isn't implementing a house rule because it isn't 'core' (in most cases, the DM refusing a house rule the player wants would have other reasons anyway, and if the DM wanted to implement it, but was hesitant about it not being core, would just playtest it and decide to remove it if he didn't like it). So, in the DM case, it's more of a 'potential shortcoming', but then again, a DM that hugs close to the rules would have been less likely to houserule with or without the suggestions, so at least the suggestions may push them to try and find something that suits their DM style, and their groups playstyle, better than just flat core.