I thought this time the theme didn't have a big role. I didn't feel like I was Han Solo or anything... And I didn't really bother seeing what the others were.
Left4Dead is basically based on all previous zombie movies. There's not much to understand about the game either... Just that there are survivors and a lot of zombies.
Hey, this would be a nice twist: innocent people are zombies, while the mafia consists of the living survivors.
Edit: for the upper link, just search the Happy Hour for 'mafia'. The threads will be on the 3rd page.