I've noticed that, despite there being a lot of online interest in the older editions of D&D, one edition seems to get no love: 2nd Edition AD&D (2e). Original D&D (OD&D), Basic (BD&D) and 1st Edition AD&D (1e) have thriving fanbases and retroclones, while 3rd Edition (3.x) is still effectively in print through Paizo's Pathfinder. 2e, though, seems to be the red-headed stepchild.
I think that a big part of the reason for this is the very conscious and deliberate division between old-school and new-school fanbases. For old-schoolers, 2e is part of the post-Gygax era, when the “grit” and the “heart” had gone out of the game. For new-schoolers, 2e is still archaic: THAC0, descending AC, class/race restrictions and demihuman level limits, etc. In other words, each of the two main schools of thought sees 2e as being too much a part of the other camp; basically, it's unfashionable.
Another big part of the reason for 2e's unpopularity is that it was the basis of arguably the biggest and most blatant “supplement treadmill” of D&D's history (depending on whether you count the third-party supplements for 3.x). Volume of support is a double-edged sword, depending on individual preferences – some DMs feel like they must have every splatbook published, others are happy to pick and choose; some like to have things set out in sourcebooks in fine detail, others like to have broad strokes and fill in the detail themselves.
Personally, I ran the game for close to a decade, and by the time I was finishing up that campaign to begin a new one with the then-new 3rd Edition, I had a love-hate relationship with the system that had slid most of the way towards hate. With the passage of time, that's cooled off. I had a quick look at the books again recently, and I've been inspired to give the game a bit of a defence. Maybe there's still a bit of the love from the love-hate still in there somewhere.
First of all, the game's core systems are really quite simple and stripped-down. A lot more than I remembered is flagged as “optional”. All classes except fighter/mage/cleric/thief? Optional. Individual Initiative? Optional. Proficiencies? Optional. Detailed Encumbrance? Optional (and, by the way, there's a simpler system there too as an alternative). 2e is, without all the optional material, almost on a par with BD&D for simplicity, but with a selection of advanced options. It's a toolkit system, more so than any other edition of D&D.
Another good point of the system is the game worlds published for it, during it's era. This touches on the supplement treadmill I mentioned above, but I think it deserves special mention as a strength of this edition. So many classic worlds were made for 2e: Dark Sun, Planescape, Spelljammer, Al-Qadim, Birthright, Ravenloft, just to mention some of the bigger ones. It shows the enduring appeal of these settings that two editions and a new publisher later, Dark Sun is being rereleased as the next setting for 4e (and there are plenty of fans calling for the others). 2E critics might say that it was a mere accident that a weak edition got good settings, but I think that 2e's flexibility (the toolkit approach) made it an ideal basis for experimentation.
Many other editions of D&D are criticised for having fighters become irrelevant at high levels. That's not the case in 2e (in fact, I think 2e goes too far in limiting mages). Spells like Fireball and Magic Missile are capped in 2e, unlike some early editions. At higher levels, magic resistance (which caster level doesn't help against, unlike 1e) and high saving throws of opponents keeps the mages' offensive power in check, while the risks and costs of spells like Teleport, Haste and Polymorph Self curb some of the excesses that 3e is vulnerable to.
Despite all that, 2e is not a system that I intend to ever go back to. First of all, it has the AD&D attribute charts, which I consider to be a succinct example of everything wrong with AD&D – they're ad-hoc and cobbled together, adding more complexity for little benefit to the game – but worst of all, they give heavy bonuses for the very top ability scores and little or no bonuses for scores just a few points less, widening the gap between characters in the same group rolled up the say way. The second reason is bloat – the D&D Rules Cyclopedia (BD&D's final version) does most of what 2e does (and many more things 2e doesn't) in a single book about the size of each of 2e's core rulebooks. I know that one of the things I got fed up with about 2e by the end was carrying around three big core rulebooks, at least one other hardcover (High-Level Campaigns) and a selection of the Complete Handbooks, plus all the Forgotten Realms stuff; and that was just the basics.