L4D looks like another genre changing experience ala portal? I can't think of another game that will play like L4D is going to be played; yeah, I know there's zombie games ::sh-duh:: but is there another game where 4 players play together as the 'good' guys, and another 4 players work together as zombies and also the zombie players get to control the boss monsters, too? w00t.
The biggest issue this game is going to have is getting people to play as anything other than zombies!
zombie 1: what did you have for breakfast today?
zombie 2: brains.