There were lots of things that drove me away from 2nd ed ( and into White Wolf, Champions, CoC, and other systems).
But here are a few things that really just drove us away.
THAC0. This wasn't bad. Once you learned it, you learned it. The problem was the -10 AC max. Played in a lot of high level games where the Fighter hit AC -10 on anything but a 1 (as a 1 always missed). Rolling to hit really wasn't needed.
Level Limits. This was a major "Balancing" Factor for Demi-humans. Problem was that unless your game went to level 12-15, the "Balancing" never occured. And, say you have a 25th level cleric, running around with that 12th level Dwarven Rogue... Oops, sorry he commited sucide around Cleric level 18, so he could make a Human that would actually gain XP.
Combat rounds were 1 minute long (generally, some books started trying to change the combat rd, which really messed everything up).
All in all, 2nd ed was just falling behind on the RPG curve (relying on individual DM's creativity & impartiallity to keep everything tip-top).
Throw in a sourcebook a week. Poor QA controls, playtesting, etc...
You have a system that was bloated, confused, over (or under) powered, and in need of some major changes. (Not saying 3rd ed was the best or only change, but a maajor revision was needed).
And Skills & Powers wasn't the answer (just move the munchkin dial to 1,000). Says the DM who had to deal with the Elf Archer with a THAC0 of 5 and 6 Attacks/round at level 3.