ESRB from destroying a game like MadWorld, built as it is almost entirely around ridiculously overblown, graphic violence? By warning them, mostly. In order to avoid the dreaded, sales-killing AO rating, Platinum Games and Sega are keeping the ratings board apprised on the development of the game, sending periodic preview builds and getting feedback.
While Platinum is ensuring the maximum allowable violence for an M rating here, they aren't seeking a release anywhere else at the moment. That basically ensures that they can tune the violence precisely to what one ratings board will allow rather than having to worry about many. If there's enough money to be had, Sega will look into other regions, but right now, it's totally US-focused.