FormerlyHemlock
Hero
1) The settings, obviously.
2) The sheer volume of supplements. Yes, I know it drove TSR right into the ground, and will never be duplicated. But all of the different boxed sets, and glossy red, blue, or green handbooks with high production values...I do miss those.
3) The three Player's Options book. I remain firm in my belief that they improved the game for the better, and that all of my best AD&D 2e games leveraged those books to the utmost.
YMMV. The Players Option era was what made me quit AD&D. Couldn't stand them.