Little kids. Who you can harm.
End of story. For some reason everything else pales in comparison to hurting a little kid- if it's "rape a bunch of women" or "shoot one kid in the face", most people would choose the latter for their videogames (ignoring that they're probably guys who just want something to get off on). Or killing innocent old people- the ones who aren't pointing a gun at you or totally evil bastards. The kind-old-lady/man type of old people.
Or a war game where you save some survivors from the Nazis or the Russians or the Arabs or the insurgents or the Government or the Pillsbury Dough Boy or whoever and upon finding out that they're German/Russian/Arab/ex-gov/pastry fans you just kill them outright, preferably whilst one/some/all of them is/are crying and screaming in horror.
So basically, any depiction of what war is actually like. Shoot someone and they start screaming and begging for mercy, but you shoot and kill them again. Killing POW's, killing civilians, etc etc, but only so long as you have to actually be there to see them die.
Because it doesn't matter how realistic the violence is, really. It's how the person reacts to it/who they are. A little kid getting killed, even if they're armed to the teeth, will be seen as worse than some middle aged guy walking down the street who gets killed. A marine- a normal marine- who's shaken from the moment you close in and who begs for mercy will be more real and more frightening than a burly chunk of space marine thundering towards you who begs for mercy after emptying a clip at you and being downed. God dammit, that guy was trying to kill you.
Reactions > gore. Hell, you can have no blood, as long as the people are beaten senseless and begging for mercy/their family is watching/etc, and they are preferably non-combatants, then you've got controversy that Jack Thompson only wishes he could hold a candle to. Scratch that, more controversy than a party where Religion met up with George Carlin and had a nice long talk could hold a candle to.
Or you actually play as the Nazis/Russians/Arabs/Government/Ghostbusters/etc.