Lots of "killing"
My son's friend came over with his Ipod and soon the boys were playing together in a Call of Minis world. While I prefer they play games where they can play together in the same world, as I sat nearby the boys' running commentary started to sound like they were playing a shoot 'em up game where the only objective is to kill or be killed. Indeed, I found the review here that said the game is basically Call of Duty cartoon-ized for kids. The analogy of the Joe the Camel ads (masking adult content to appeal to kids) seems right on.
I know these kids at age 9 are younger than the game's age rating, but I have a 13 year old who I also wouldn't let play. There are plenty of creative, fun games available where the objective is more thoughtful than "kill or be killed."