My first encounters with D&D were the original AD&D, however my first set was actually the boxed set of D&D, often called "Basic" D&D. This was a decent introduction, especially if you didn't have anyone around who already knew how to play. However, it had some oddities. The biggest one was that race was simply a class options. At the least all Elves were Fighter/Magic Users. I don't recall if there were any other races or not.
The original AD&D, now referred to as "First Edition" -- much like the original movie released as Star Wars is (at least supposed to be) referred to as Star Wars Episode IV: A New Hope -- was a very good game. But when first released, it was designed for the kinds of adventures common those days: dungeon crawls with the occasional wilderness variant and some town time to get it started. Role playing did happen, but most of the reward came from killing monsters and getting treasure, which usually happened in the dungeons.
Originally, it was contained in three books: The Player's Handbook, The Dungeon Master's Guide and The Monster Manual -- often at least in my circle of friends called "The PH," "The DMG," and "The Monster Manual." After several years, TSR started releasing supplements. The first several were added references, mostly new monsters, such as The Fiend Folio, The Monster Manual 2 and for gods and heroes Deities and Demigods.
It was this last book that got TSR into a bit of trouble. They included two sets of "mythos" that were derived from copyrighted works, without appropriate licensing. I suspect that, even in its far from mint shape, my "DD" may be of higher value than some of my other books since it is the true original printing. However, I've heard that there were versions that had had the problem pages cut out, which may be worth even more. But, I'm not planning on parting with any of my books any time soon, so I'm not that concerned.
For the most part, my friends and I played the kind of games that the rules best supported during this era. We also played in other systems, mostly in other genres, and occasionally made house rules, or even full adaptions, for different experiences.
Then TSR issued, in fairly short succession if I recall, two supplements that added new classes: Unearthed Arcana and Oriental Adventures. The later also introduced the idea of a non-weapon proficiency which, for the first time, gave characters testable skills outside of those shared by all members of their class. Two books that came out a bit later further expanded on that idea by making them available for Occidental adventures as well: The Wilderness Survival Guide and The Dungeoneers Survival Guide. Other supplements, many for specific worlds (Greyhawk, Dragonlance) or alternate adventuring options (The Manual of the Planes) were also released.
At this point, AD&D had grown from a game suitable for the early style of gaming into one that also supported long themed campaigns, character interaction, and distinction between PCs beyond their stats and played personalities. However, it had also gotten to be out of hand with the number of places one had to go for any given piece of information. For example, character generation could require looking up information in 4 or more books. There were also conflicting rules and guidelines. So, TSR -- in my opinion fairly sensibly -- decided to release 2nd edition AD&D.
At least from my perspective there were few major changes between 1st edition, at least just before 2nd edition came out, and 2nd edition. However, they streamlined the rules and removed some of the conflicts. They also changed the reward system from strictly based on how many monsters you kill, into one that the DM could use to reward the kind of play that they wanted.
Even though I was about to graduate college at the time, I jumped into 2nd edition and used it to ref my last campaign before leaving New Mexico (one we had to rush to finish so that I could leave). It was also the basis of most of the games I ran over the next few years for various reasons, largely the preference of the other players. In fact due to the other changes that impacted my life, and my friends' lives, starting in the mid-nineties my last regular reffing was done with 2nd edition AD&D, but my last regular playing as a player was 1st edition since that was what the other ref preferred to use.
As gaming companies tend to do, TSR released several supplements. They, as near as I can recall, refrained from releasing any more main books, but chose to release smaller supplements often expanding the options for a given class, but working mostly within the established rule base.
During my time away from the hobby, TSR was purchased by Wizards of the Coast (who were subsequently purchased by Hasbro). After their purchase, the decided to release a 3rd edition of D&D, dropping the "Advanced" part of the title. I had very little exposure to 3rd edition, but eventually got the books for the minor revision called version 3.5.
Having not actually played -- unless you count running a solo fight that nearly wiped out the party of "PCs" that I threw at what I thought was a small company of Orcs -- that version, I cannot comment a whole lot. However, I do see that they did a few sensible things to streamline (again) the rules. A key fix was to make most of the roles d20 based, and to make all of the d20 roles go the same way. With 1st and 2nd edition checks against a raw stat, or closely related checks such as non-weapon proficiencies, were a role low check. Combat was, on the other hand a role high check. They also fixed the need to understand math with negative numbers to figure out the number needed to hit (in first and second edition, the number needed to hit an opponent was THAC0-(target Armor Class), which was often a negative number)*.
They got rid of the idea of multi-classed non-human and dual-classed humans by making it possible for every character to train in a different class at each level. Having the experience necessary to go up a level the same between all classes helped with this as well. I suspect that this may have hurt some small, low level, parties that relied on multi-classed characters, but probably produced a smoother game overall.
This last year, following on in the timing and traditions established before, WoTC released 4th edition. I've written before that it seems to be aimed at making paper and pencil role playing comfortable to people with experience in console or computer role playing. But the result is the least D&D like of the rules. Based on my still limited experience, I may continue to play, primarily if I end up doing more RPGA LFR gaming just to do it. But I'll probably use either 3.5 or 2nd edition if I want to actually ref a campaign.
Of course, since I don't have much spare time (and I'm about to offer some of it to either ConDor or Conjecture or both), nor anyone around here that I'm likely to ref for, it may be a while before I actually need to worry about this.
---*An aside on THAC0
On the Christian Gamers' Guild e-mail list recently, someone railed against all editions of D&D beyond 1st edition AD&D. One of their key points was that they hated the concept of THAC0. However, when I first encountered it was well before 2nd edition came out and made it official. (I think it may have been on the pre-generated characters for the Dragonlance modules, so it was almost official). But once I understood what "thack-oh" stood for, I realized that it was almost no different than what had been in AD&D from day one, with two very minor possible exceptions. First, it was no longer a secret number that the DM kept and read off of a table on his screen or in the DMG. Second, if the players followed the formula