I only have a few suggestions. Some of the other elements I have in mind are likely to come with a revision anyway, such as simplification of attacks of opportunity.
A. Full 20-level support for any class presented in the Player's Handbook. The fighter is very dull at high levels unless she masters multiple fighting styles; there are no worthwhile high-level warrior feats in the core rules whatsoever. Similarly, sorcerers have nothing to keep them out of prestige classes - not even the bonus feats of a wizard. Sure, how you roleplay a character can provide enjoyment even when they're mechanically dull, but why not have a mechanically-interesting character and roleplay them well?
B. If there are to be races which are more powerful than the "baseline", then they need to actually be more powerful than the baseline. All playable "monster" races should have "Level Adjustment"-type abilities firmly tied to racial Hit Dice - in essence, there should be monster classes with Hit Dice at every level, not every two or three as they are now. This necessarily entails a redesign of these races' power, but that's no big deal - drow have always been more powerful than normal elves, it's just that Third Edition quantified that power into level equivalency. What I'm proposing is true level equivalency.
C. Magic does not have to be unified into a single type - and I feel it shouldn't be - but there needs to be a way to make multiclassed magic-users effective in a way which goes beyond just "+1 caster level" or the like. This ties back into the previous point: races more powerful than the baseline with an affinity for a spellcasting class ought to actually unify with the class they're supposed to be good at, so as to prevent the foolishness of a wizardly race with a Level Adjustment which means they're always looking like lower-level, weaker wizards. To use the drow as an example: give them spell-like abilities which would be useful to non-spellcasters, but have those abilities exchangeable for effective levels in an appropriate spellcasting class, explaining it as a refinement of their innate gift for magic away from the uncontrolled manifestation of their roguish or warlike peers and into a mastery of the full scope of magic. These should be truly effective levels - think of them as substitution levels for the class in question! Give them different class features, which differentiate them from (and balance them against) "baseline" races.
D. Make each class distinctive and interesting. The wizard and the sorcerer are too close together. Replacing the sorcerer with the warlock, with more "flavour templates" than the "chaos & evil" default, is a start.
E. For slot-based casters, use the Arcana Evolved simple/complex/exotic spell divisions, spell descriptors, and spell slot splitting/combining methods. This makes the third point above easier: better-than-baseline spellcasting races can be balanced in the "substitution level" fashion by access to some spell levels and not others. For instance, a drow's effective cleric levels could be restricted to certain drow-appropriate descriptors as a balancing measure. Provide a mechanic for "catching up" - say that a drow's highest two spell levels are always restricted to simple spells and complex spells dealing with poison, darkness, pain, whatever, but that their complex spell options open up at lower spell levels as their mastery of lesser magic increases. "Baseline" clerics will thus always have an edge to compensate, but drow clerics won't be underpowered.
Just some thoughts. Could be pretty complex, of course, but I don't think D&D needs to be super-simplified. Reward system mastery.