Valve chose New Orleans as L4D2 location because it's different
Valve's Chet Faliszek has said that the reason it chose New Orleans as the location in Left 4 Dead 2 had more to do with the east coat of the US not being used in games much, this making it a fresh locale.
"We still wanted it to be on the East coast, a bunch of us are from the area," he told Planet Half-Life. "It's an interesting place that is not covered in a lot of video games. There's not a lot of video games that go down to the deep south, through the Savanna and New Orleans. There's a lot of cool gameplay there, there's a lot of cool areas. You can be in the swamps, the Savanna or New Orleans, it's a beautiful city.
"It seemed like a good opportunity to go through there, so we jumped on it."
Don't forget that the deep south can be a very mysterious place, what with all the live oaks littered with Spanish moss fluttering in the breeze; the humid heaviness of the air; the thick essence of jasmine and wisteria wafting through your nostrils; toothy zombies just gagging to sink rotting fangs into you.
Great place it is.