Once photorealism kicks in, i can see violence affecting people alot more, than it does normaly games.
Max payne is quite violent, however, it is only because of the graphical level that the violence hits you.
with gta, you can mow down hundreds of people, yet it not affect you, yet after a couple of brutal kills in MP, it can.
Maybe the story is portrayed better, and you feel like max, i dont know, but i think its down to the increase in graphics, and what the can show, e.g. blood dripping and flowing from a bullet wound, or the slow motion shooting.