One of the questions I get asked a lot is why I am playing OD&D/Swords & Wizardry rather than AD&D. I think that's a perfectly reasonable question, because I've admitted on numerous occasions both my fondness for AD&D and my willingness to use a lot of the supplementary material for OD&D, which brings the game well within spitting distance of its descendant. Given that, why not just go whole hog and play AD&D?
I have a couple of responses to this question, but the first one is that I simply prefer the lack of assumptions that comes with playing OD&D. Were I to play AD&D, it would be "AD&D minus," which is to say I'd be excising many elements from the published rules and would have to make a point of telling my players what parts of the game I wouldn't be using. When I play OD&D, I play "OD&D plus," where I tell my players what things I'm adding to the three little brown books.
Now, this may seem like a distinction of no consequence and perhaps it is. Yet, I can't deny that, from my own perspective anyway, playing a "plus" game is conceptually simpler than a "minus" game, since there's no confusion in reading about class X or spell Y in the Players Handbook, getting all excited about it, and then being told by the referee, "Sorry, I don't use X and Y in my games." One of the reasons I've been so down about supplements is that I feel they often create expectations in the minds of players that put undue pressure on the referee to accommodate them. Certainly the mere existence of a new class, spell, or magic item doesn't put a gun to a referee's head, but I know from experience that many players nevertheless assume that, if it's in an official game book, it ought to be in the game too. I have very reasonable players and yet I still wish to avoid that.
The second reason I'm not playing AD&D is that I think the baseline level of character ability in AD&D is much greater than I feel comfortable with. Hit Dice are all greater than in OD&D, for example. Ability score modifiers are all generally higher. There's also the explicit assertion that characters who lack scores of 15 or higher in two or more abilities are somehow sub-par. There is throughout the game a subtle but pervasive power inflation compared to OD&D and it no longer suits my tastes as far as fantasy gaming goes. That's not a condemnation of AD&D's approach by any means, but rather an acknowledgment that, for all the tweaks I am making to OD&D, I'm still a good distance away, both mechanically and philosophically, from it.
And that's the long and short of it.