I've been browsing the net to try and find some more interesting ways to arrange classes and levels. I have a flu or cold or something, so I am allowed to waste time on this.
The E6 System
It's pretty neat. The gist of it is that D&D 3.0/3.5 is much better if you stop at level six. After that you just get feats.
I agree with the creator in that un-altered d20 d&d gets a little complicated after around sixth or seventh level. The makers of 4e agreed, too. One of them called it the "sweet spot" and tried to spread that out over the entire 1-30 level stretch. I don't think they succeeded, because 4e kind of collapses under its own weight of extra attacks and immediate power gewgaws after a certain point, especially if you have more than four players in a party. The more people there are, the less interesting characters can be, strategically, or combat slows to a crawl and never recovers.
Some of the simplest fixes to 3.5 is to change all the durations away from rounds or minutes, to eliminate pre-casting, and to have a hard limit on the number of spells any spellcaster memorizes each day (down to like, maybe ten spells that can be cast twice each, perhaps). Those alone fix a huge number of problems with 3.0.