City of God and those other films have violence, but I think the difference is that City of God has teens and younger doing much of the killing. It's one thing to seen grown men slaughtering one another. It's something else when you've got a ten year old shooting a five year old from close range.
I did my part for Punch-Drunk Love, but I knew it wouldn't be enough outside of my little circle. Too quirky, too unsuccessful at the box office, and released too early. ;)