The current "abstact" system already assumes that you are parrying. Do you know why you get a base AC of 10? Because your character is "Taking 10" on his defenses. (I believe I've read this somewhere before, flame away if I am mistaken).
That base AC of 10 assumes your character is blocking blows and moving around. He isn't just standing still in his little 5' by 5' square, he moving, blocking, parrying, and trying to land blows of his own.
This is the reason why precise shot exists. If a person was actually standing in a 5' by 5' square and fighting a guy 5' away from him, he wouldn't have a big penalty to hit. That is because the rules are abstract. D&D isn't meant to be blow by blow.
Now, I've fooled around with several parry options myself. The end result is this, IMHO. Combats are longer, critical hits become the only attacks that hit, and skilled characters never take damage. I'm all for the blademaster that can almost never be hit, but making ten rolls for five seconds... It got boring and labored quick. Parry mechanics really start to break down at higher levels.
I used to think AC was a crock too. Now I see how fluid it is and why the designers made things so abstract. Speed. You can resolve combats quickly (keeps the feel of that six second round).
If you want a more complex system, I say go for it. The NetBook of feats has lots of parry options. Just be prepared to face the re-balancing issue with the entire combat system. (Player: Why can't I block that Magic Missle with my shield or magical power sword +5?)
Good luck in whatever you choose to use.
