I'd say "Heeeere we go again." but I don't think this time the thread will get too big... unless....
If you want called shots.. I want called defense. There is absolutely NO reason why someone should be able to say "I try to hit you in the face." and someone else should be unable to respond "I try to raise my hand to stop you." That's basically what attacking and defending already are.
No reason save those already addressed in the mechanics of being denied my dexerity, or flanked by a rogue.
Unlike Combat Expertise and Power Attack, things like Called Shots depend upon a degree of ineptitude on the part of the defender as well as skill of the attacker. Without any other influence, a fighter can opt to swing harder, or focus on being defensive... but when you say "I try to out do you for special effect" there are two sets of variables that have to be addressed.. the attacker's competency/luck and the defender's.
Fine by me to do this... but it's almost pushing into Storyteller system mechanics to have attack and defense rolls. Again.. fine by me. But no "Oh i take -2 to blind you." stuff... not without "I take -2 to my next attack to not be blinded." at least.
There.. let the fun begin.