If I absolutely had to cut something it would be either Attacks of Opportunity, or Prestige Classes...although neither in their entirety.
AoO just need to be set out more clearly, and trimmed back a bit so as to not to dominate combat so much.
Prestige Classes would be fine, if the various designers had listened to Monte, Jonathan and Skip in the introduction of such things- they should be PRESTIGIOUS, and have a special significance. But nooo...instead we see every Tom, Dick and Harry putting out umpteen thousand of the things, hell, even entire PDF companies who's sole reason for being is to pump out stupid PrC after another...
Phew...sorry 'bout that, I'm better now.
Honest.