I always thought it would be cool to have a game complex enough to have calculations of physics as well as biology, it would calculate whether or not a particular blow to a particular area of the body would cause an injury, and that injury and its effects would be realistic. It could calculate whether a blow to someone's head is enough to decapitate them, or how long someone would have to bleed before they died. (A multi-player game should allow for respawning if you suffer enough damage, so you don't have to wait out a match lying at the bottom of a shaft with broken limbs, unable to move, or if you get hit in the head too many times and lose all your senses but are still alive.)
Of course, every anti-violence group would totally be against that, as anyone could then use the game to "train" people to see what can or can't kill a person. o_0
I'm sure that even if it has zombies with real-time decoposing skin, and if the game is any good at all, people are going to make a big deal out of it.