Here's my basic premise: the more dice that are rolled in a round, the longer combat takes, and the more clunky it feels.
Honestly, on some levels I feel that 2nd edition had it right. don't tie the number of attacks you get per round to your bonuses, but to your class levels. only high level members of the warrior group got in 2 attacks / round, and it never went beyond that.
I think that multiple attacks per round should be a class perk, somewhat like weapon specilization. I also feel like significantly reducing the effectiveness of two-weapon fighting by dropping it back down to only one off-hand attack per round, period, is a good idea. I feel like a 20th level fighter should be very happy if he manages to get four attacks in one round. perhaps making multiple attacks linked to a feat that requires a certain base attack bonus in order to get.
The BAB mechanic itself is basically a thac0 progression, just presented a little differently. It's a solid simplification mechanic, and I definetly like it. I think that adding fractional BAB into the game as a standard rule (from Unearthed arcana) would help the viability of multiclassing. I like the unified "roll high on a d20" concept - keeping that around means keeping ac in the positives and keeping BAB as an ascending bonus.
In the same vein, I think reducing the number of possible attacks of opportunity in any given round is a good thing - specifically, I think that removing all AoO's tied to movement would significantly reduce their impact. leave them in for spellcasting, attacking with a ranged weapon while threatened, and standing up.
Willpax - I think that, given the ammount of damage high level fighters dish out, it's a bit redundant togive them even further damage bonus. yes, the kind of changes I'm talking about do require a massive game rebalance, but I'm considering it in a 4th edition kind of way - massive rebalance will be needed anyway. Since I dislike the standard/move/swift/free action method of dividing up a round (as it encourages the stand back and prepare an attack when the enemy moves into range philosophy), reducing the number of attacks per round but making it a basic perk of high level fighting classes seems like a decent compromise.
Sobek - I'm quite willing to talk about AC as DR, but on some odd level I like the hit points mechanic, as opposed to WP/VP. what do you have in mind there?