My take on the editions is probably a little skewed by my experiences with them, but here goes:
I skipped AD&D 1e. Never played it. But from what I can see, it's OD&D plus supplements, just codified and streamlined by Gygax. Which means that it's crunchy and tactical. The complex combat system just doesn't work like it's supposed to if you don't use all the tables for weapon speed, weapon types vs. armor, initiative segments, etc. And I wouldn't want to try and play this edition without minis and a grid; it'd be a nightmare.
AD&D 2e was probably the first system I used for long campaigns. The rules were sparse and they encouraged house-ruling and homebrewing. Ours was not a group that used published settings, so whenever anybody says that 2e is about the settings, my immediate reaction is something to the effect of: "Hell yeah, it was about settings! I must've written, like, five of 'em. 2e was great for that. Oh, wait, you're talking about Realms and Dark Sun and Spelljammer and all that crazy stuff? Never mind." We garnered stuff from the splatbooks when it was useful, ignored everything else, and just used 2e to engage in some roleplay-heavy D&D. Combat was incidental, and often avoided with sneaky tricks and clever ploys. We also just accepted the quirky rules (like exceptional strength and non-weapon proficiencies) for what they were, tinkering only to add things to the game that seemed missing. And combat was done largely without minis, in a Final Fantasy style back-and-forth "I hit. You miss. Fire2 *cough* I mean fireball just damages all the enemies on one side of a combat."
3e came out, and suddenly we were all conscious of game balance and positional tactics. Our combats went from feeling like Final Fantasy to Shining Force---which was, IMHO, a marked improvement in combat, since it made that part of the game way more fun---but since 3e combat is so *slow*, fights basically took up 80% of every game session, and the roleplaying was really left by the wayside. When 3.5 came out, this edition had by then become way more bloated than 2e ever was with crazy splatbooks, it was an absolute pain in the keister to DM, and whenever a new player (raised on 3e, of course) joined our group, they would complain during the "talky parts" and whine about how they wanted to get into a fight.
So, about a year or so before 4e was announced, I had already ditched the d20 system for OD&D. Specifically, I went to the 1991 version of the game, the Rules Cyclopedia version, since it was the first RPG I had ever played. (2e was my go-to system for long campaigns, but the RC was the book I used to pull out for dungeon crawls.) Having matured as a DM and experienced many crunchy years of 3e, I've learned what I like about roleplaying games and what I want out of DMing them. I want lots of heavy "talky" roleplaying, and a combat system that uses tabletop tactics but won't take up the whole play session. OD&D is great for that, whether you're using D&D '81 (B/X), '83 (BECMI), or '91 (RC).
Once I started DMing with the RC again, it was like I had my gaming soul back. The 3e rules had been like an intricate machine, and as players and DMs, we just operated the machine. The OD&D rules are a canvas with a few sketch marks on it. They lend themselves inescapably to improvisation. It was teary-eyed joy to once again run games where the players absolutely *had* to be clever and quick to succeed. But at the same time, for all the wonderful memories I had of 2e and creating epic fantasy stories, I wouldn't ever want to go back to that edition, because it would mean sacrificing good combat rules. 2e combat is quick, but way too sketchy for my taste. OD&D retains some of the crunch featured in the combats of its cousin game, 1e. As it turns out, the best way to play OD&D is far away *with* minis and a grid, but it remains quick and elegant. It's the system for me.
From what I can tell, each edition did indeed have definite and different goals.
- Early OD&D ('74 white books, supplements, '79 Holmes basic) and AD&D 1e were basically the same: this was a wargame with story and roleplay elements. The goal of the game was to get in, get the treasure, get out alive. It was only later in the 1e era that the story elements began to be noticed and fully realized, and Gygax himself really seized on this in his later writings, heavily presaging 2e IMHO.
- Late OD&D (B/X, BECMI, and Classic/RC/WotI) was a gamer's game. It was like the super-simplified cousin of 1e. Get in, get the treasure, get out alive, and we're not going to give you very many rules to tell you how to do that. The best part of this edition, though, was that it actually had rules for high level play. 1e's goal was "reach 12th level and retire to your castle or tower." With this version, the goal was "adventure until you reach 9th-12th level, build a castle or tower, become a noble, instigate some wars, hire some spies, conquer other territories, become a king, go exploring, conquer other nations, build an empire, transcend legendary greatness, quest for immortality, ascend to immortality, and start frelling with the cosmos as you see fit."
- AD&D 2e was a storyteller's game. Rules were sparse and often optional. Combat was loosely defined. The point of this game was to tell a good fantasy story, with as few constraints on the players and the DM as possible. Whatever RPGs I play in the future, I will always take away the 2e style of storytelling and campaigning with me. It's difficult to do this properly in 3e, but it works well in OD&D and probably 1e as well.
- 3e is another gamer's game, and tactical to boot, but very complicated. Rules aren't optional or modular---they're difficult to ignore. The game is necessarily heavy on combat most of the time, but it's also really fun. I'll always carry that with me in my future DMing as well, and I find that it's easy to use 3e style combats in many simpler games with fewer rules: OD&D, 2e, even White Wolf games.
- 4e, from what I've seen, is problematical for what it's trying to be. It tries to solve all the problems of all the past editions by using the rules to define things in certain ways. Something you can do in a fight is now a "power," something you do outside of a fight is a "ritual," something mundane but useful in a dungeon crawl is a "skill," and everything else is ignored by the rules altogether. I'm very ambivalent about this approach. I honstly can't tell whether the goal here is to be a tabletop wargame, or a fast-and-loose, frestyle roleplaying game where the rules are only called upon when needed. It might work either way depending on the style of indiviudal players and DMs, but right now, I'm guessing it encourages and leans heavily towards tabletop wargame. So... I'll play it if somebody ever offers to DM, and I'll bet it's darned fun, but it doesn't inspire me at all as a DM.
