AoF3 is an awesome game. It only feels "stiff" if you play it like KoF since it's counter-based (like the aforementioned Virtua Fighter), but that statement alone doesn't do justice to AoF3's elaborate engine.
I've seen many ppl playing it for the first time, and most of them immediately try to do some qcf's or hcb's+button moves, but IMO, AoF3 doesn't live off of these standards but of string combos, fast counters and a strategic use of supers.
neobyer mentioned the motion captured anims, and I think this is where the "lags" and stiffness come from; most charas in AoF3 do -complete- movements, not the type of short-cut animation other fighting games have.
Take Ryo's standard qcf+A projectile. If you do the move, it will take nearly two seconds before the fireball comes out. Ryo puts back his arms in a smooth round movement, builds up energy and then moves his arms forward to throw the projectile, all in one nicely animated sequence. This takes a lot more time than his KoF flame punch, so doing the move during a close up situation will create a hell of a lag and more than ample chance for your opponent to hit you.
If you insist on this style of playing, AoF3 will seem slow and jerky to you, and you will never be able to beat it. To solve this, concentrate on the string combos, don't do too many jump attacks and only do supers if chances for connecting is high or chance for immediate retaliation is low (opponent is a screen away/in a juggle/knocked down, etc.).
I think there's hardly another fighter with an engine as deep as AoF3's. I like technical fighters, and if you do too, you will love AoF3.