There was only one change that I felt was "required" for 3rd edition, and that was the problems with high level play. My DM in most of his campaigns had an implicit "level cap" around 9th-10th level because he thoroughly did not enjoy the work and pains of high level gaming.
However, that is not a single change, that is the result of a whole host of 3rd edition mechanics. You can't just flip a switch and fix that problem, you have to change a lot of factors to make that work.
So I guess ultimately the question is, "could you just tweak 3e and make that work?" I don't know, I've never tried. I think E6 fixed the problem with high level play...in that you never get there, but that's a solid variant. But I've never seen mods or tweaks to the system that really fixed the high level issues, and from I've seen of pathfinder its not shaping up to fix those either.