All what I would like to see is that if you shoot somebody between the eyes he dies instantly, if you make an accurate shot to the arms he cannot use the arms -> cannot shoot, if you shoot him in the leg, he cannot walk properly. That is all. I didn't see this yet in any FPS game. But would very much like to see something like that in ArmA.
And one more thing, I would really like to see that one is required to use the sights to make the shots and not just have a crosshair on the screen.