3e changed the most rules. The biggest change in those rules however was changing it from D&D to Rolemaster Lite. Made a TON of Ex-D&D players who hated classes delighted to call a rolemaster like game...D&D. Not that it mattered, the numbers playing AD&D had shrunk significantly by that point. There were more Ex-D&D players than active and admitted AD&D and D&D players combined.
HOWEVER...the BIGGEST change which part of this post I put in that thread (so if you read it...I'll put a divider where I have other things to say). It's pretty hard to say whether it was between 0D&D and it's supplements (Greyhawk changed the world and how it was run...almost literally...many wouldn't recognize 0D&D as the D&D today...they would recognize it after Greyhawk though).
Something else that was a slower change, more in the minds of players rather than the game system was between 0D&D and AD&D as it came to be in the mid to late 80s. Originally, via the wargaming ideas, characters that were low levels were exceptional. They were better than many of the other monsters and people of the world around them. This idea was even found in BECMI...as you can see...a first level fighter isn't some new novice...he's a freaking veteran. They are trained and experienced.
However, somewhere along the way, they became freaking newbies just wet behind the ears...not really trained, and ready to die. This is exemplified by 3.X's take on what a character starts off as.
The older thought process had the idea that it took years just to get to 1st level. Because of that, it was incredibly hard to get levels in another class you didn't start with.
In 3e it was a matter of a simple decision at a level up...or became that. In some ways 4e, with it's feat system has made a return to having the heroes having a slightly tougher time training...as well as starting the heroes as deadly trained warriors instead of some newbie fresh off the boat who hasn't seen a sword before a month ago when they finally got trained on it.
Another massive change was AD&D/0D&D supplemented (that's with the supplements inclusive of Greyhawk, etc.) and BX and BECMI D&D. Races that were classes (even if they simulated what was played previously) and some other items were pretty big changes.
The biggest concept change was probably 3e however. It changed from class as solely the archtype, to where class could be a symbolic thing you dipped into. This of course is a rules change reflected more by the change in through processes in previous editions rather than the rules.
The rules changes are numerous however in many of the editions. Between core 0D&D (without supplements) to AD&D would perhaps be almost equal to the number of changes between AD&D 2e core and 3e. Hard to say though.
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4e is actually a refined version of 3.X edition. It's hard for those who started with 3.X to actually see that, but 4e keeps the core ideas of 3.X going strong. It uses the ascending AC and a whole host of other things from 3.X to the point that it's EXTREMELY EASY TO RUN a 4e game as a 3.X game. In fact I once pushed some rules ideas for those who wanted to do such a thing...but got it shot down by people stating if they wanted a 3.5 game, they'd play 3.5...they didn't play 4e simply to rehash an older edition.
Because of that, I see those stating 4e as the biggest change...rather deluded by how drastic the changes were for 3.X and it's differences to previous D&D editions.
~ so anyone who disagrees with you is 'rather deluded' and clearly incapable of critical thinking? That's an exit from the thread for you. Plane Sailing, Admin ~
If anything, 4e took a step BACKWARDS towards what older D&D editions were, with as mentioned previously, making class once more heroic at first level (though this was as I also previously mentioned, more of a change of mental though...with the heroic trained veteran an idea originally given...but changing in the minds of players...rather than the rules...at least prior to 3e...to one of a guy fresh off the boat and wet behind the ears later on), and making it harder to change classes. The archtype once more became king.
4e ADDED to the already onerous rules of 3e in some ways. Thief skills remained among the skills of players...just like 3e...and feats were still included...just like 3e. However 4e added powers for EVERY class.
3.X had started the change in vancian casters (just like 2.5 had started the change in class as an archtype as classified in rules rather than just in player's minds). First up you had an increase in the actual number of spells a Wizard could cast. Wizards originally were pretty limited with their number of spells at low levels they could cast per day. This made for many unhappy ex-D&D players. This changed in 3.X so that casters got MANY MORE spells at 1st level on up. 0 level spells actually got some oomph, and some of the original 1st level spells were among them. Sorcerers changed it even further, with even MORE spells available (and at that point, with how much some would be able to use spells, it became more a matter of why even have a spell limit), even if with a smaller selection. Then came the Warlocks...
What is more significant is the change in mindset that started to occur with some of the players. The idea that once the caster was out of spells, the party simply rested till the next day kicked in. At that point, is that really Vancian casting? It's basically what 4e does to a degree...though you'd have even MORE 2nd, 3rd, 4th, 5th...etc. level spells (though with 4e you'll have unlimited 1st level spells...well...with the number you can choose from). This wasn't really codified in the rules per se...but it started to happen...and once it did...it paved the way for the removal of a strict Vancian system (though it exists in a limited form in 4e...though you could say that with all daily powers for all classes in 4e...suddenly the fighter can't perform a combat maneuver...or a rogue forgets how he can do this little stealthy move) in 4e.
I think rules wise, 3e wins...though it's VERY CLOSE with the changes between 0D&D core and what came later (especially once GreyHawk kicks in, and 0D&D compared to AD&D...totally different beasts...I'd say as different as 1e is to 3e...maybe even 1e compared to 4e).
The differences between each edition however is harder to say simply because I think it involves more of a mindset of the gamers rather than simply how rules were written. There was a slow but steady change in how the game was percieved...so wargamers who started the hobby in the 70's would have a different perception from that point, then what others...or even they, may have by the mid 80s.
I think the big shifts of mindsets came a little after the mid 80s, and around the mid 90s. These changed perceptions of archtypes, what a class was, what a level was, what skills were, and basic conceptions about what gaming was in general. These ideas were eventually codified into rules, but the ideas actually existed before those rules came about.
Much of what you see in 3e was already practiced by houserules, or thought of as a generally accepted idea prior to them being written down for 3e. Even if I might consider it rolemaster lite in some ways, and call it D20 Fantasy instead of D&D...it really was composed of many ideas that had already been circulating amongst D&D players (and many ex-D&D players) for YEARS.
These were massive changes. 3e just codified them.
In that light, as far as codifying a changed mindset...I think 3e is the biggest change simply because it made it so that these ideas collected for the past 20 years were suddenly codified. No other edition had to collect such changes in the gamer's mindset from such a long period (4e only collected that of a subset of gamers from the past 8 years, as opposed to 2 decades. 2e didn't do as much to change the game as some suppose. 0D&D to AD&D had some major changes...as the game was still being formed in many ways...but still the timeline wasn't quite as huge as far as the perceptions of gaming).
Okay, it turned out lengthy anyways...sorry about that.
PS: And to make a lenthy analysis even longer...obviously...the biggest difference of them all is between 0D&D and 4e...the game has changed so drastically in rules and player's thoughts between them...that's it's just about as vast as the differences between 0D&D and a non-D&D game system (though derived and called D&D by many)...that of Pathfinder. I think people have a hard time determining differences between the actual edition to edition...rather than their preconceptions of what changes are in relation to the game itself. By default...since they are both on the separate spectrums of time...0D&D and 4e are the most different...that's obvious. Ironically in some ways...thought wise...4e is actually CLOSER to 0D&D than 3.X though. And people are calling 4e the biggest change...which I see as...ironic. If it's any consolation...I don't think Gygax would have preferred or liked 4e either. But that's my opinion.
For the record...I play 3.5...and I also like 4e...at least currently.