There seems to be something of a consensus that high-level play is broken. I have also frequently heard people say that the middle levels (somewhere in the 6 to 10 region, depending on who you ask) are the strongest part of the game. I see two main reasons for both of these observations. First, magic. It simply starts to get out of hand at the highest levels. Second, the ever-widening gap between 'good' and 'bad' progressions; by 20th level, the difference between a good save and a poor save is large; the difference between full and 1/2 BAB is massive. It reaches a point where, against the same enemy, a fighter can auto-hit while a wizard auto-misses. A wizard at that level shouldn't be attacking all that often, but it still strikes me as too large of a variance.
My proposed solution is changing BAB from 1/1, 3/4, 1/2 HD to 5/6, 4/6 or 2/3, and 3/6 or 1/2 HD. This means that the various max BABs are 16, 13, and 10, which seems like a much more manageable spread.
As for magic, I'm looking at a full rewrite of the spell system, with 7 spell levels, and a new one gained every 3rd level. That's a little much to get into just yet though.
Thoughts?