Hmm, know that I think about it, my more reasonable changes:
--boosted theives. Obviously too weak (and each following addition--from what we know, has/will also boosted them beside renaming them).
--gave wizards ways to cast spells when they normally would have run out (again, forward looking)
--actually ditched halflings instead of gnomes. And kept them out for 3rd ed.
--didn't give XP for gold, but ended up giving massive "role-play" XP just to keep things crawling along.
--allowed demi-humans to duel class, and humans to multi-class (and in fact had the human fighter/rogue and the elf who was first a cleric, then a wizard).
--critical fumbles using novelty die
--a perception score (hey, also forward looking, but also obvious)
And those I don't regrett. These I did, or at least changed back:
--armor as damage reduction
--defense and damage bonuses from leveling (though it was not a bad idea, implementation only so so)
--using bunch o' subraces (which continued into 3rd ed)
--way to much "official" rulage coming in from various sources.