Poor AD&D 2nd Edition! Reviled by grognards and RPG fashionistas alike! Yet, I submit from my own personal experiences that 2e still has a decent amount of groups out there who play it or occasionally cycle back around to it. In my own group, I have at least 2 players who consider that their edition of choice, and I'm fairly certain a goodly number of players near my age (near-30ish) started with that edition.
Now, some of the main criticisms of 2E were:
-The "softening" of D&D by removing mentions of such things as demons and devils
-Removal of bards, monks, and half-orcs
-Removal of dungeon generation tables from the 2e DMG
-The way Non-Weapon Proficiencies worked (or didn't)
I think a lot of the strengths of 2e were in the "fluff", settings, boxed sets, and the Complete _______ line of books--all things that wouldn't or couldn't translate to a retro-clone as well as mechanics.
Let's be honest--in a lot of ways, AD&D2e is the "Silent Years" between the Old & New Testaments of D&D for people. I think if we see a completed 2E retro-clone, it may well come from outside the current retro-clone community. The closest thing I've seen thus far was the 1e/2e mix of Hackmaster, and that's also now out of print (the next edition will be a completely new game). I think it is has been stated as a clear point of delineation by many old-schoolers, and I won't argue with that. I think the game may even stand for a different style of play then the retro-clones we've seen up to this point. But it is still a part of D&D's history with it's own charms, and it would be at least interesting to see such an effort.