The Houston Chronicle's game blog http://g4tv.com/thefeed/blog/post/697427/Is-Racism-Institutionalized-In-Video-Games.html pointed out the similarity in imagery between Left 4 Dead 2 and the aftermath of hurricane Katrina.
The game is set in New Orleans. It was only a few years ago that the city was destroyed, and there actually were corpses moldering in the streets... so it's natural that the game's visuals might remind people of the tragedy.
Left 4 Dead writer Chet Faliszek spoke to http://www.destructoid.com/valve-responds-to-left-4-dead-2-racism-accusations-141921.phtml about the accusations, to quote:
"Utter insanity," he said. Faliszek. "There are mixed races of zombies, there are all different races of zombies that you shoot, and since we placed it in New Orleans, that makes it racist? ... When two of the characters in your game are African-American, it's a weird thing to be accused of. We're like, 'how does this work'?"
"As far as Katrina goes, if you go down to New Orleans, Katrina's still going on. I mean, it's messed up, it is crazy that the city is still in the state it's in, and we treat that with the utmost respect...This is a videogame, those are real people's lives, we are not trying to make a statement with that ... It's a place we love, it's dear to our hearts. We would not cheapen it. It's not a brick-for-brick representation of New Orleans; it's a fictional version, and I love that city."
i think that some if not most of the people that say that the game contains racism are a little racist themselves. they really looked for the controversial parts and reported them. i think it takes one to know one.