Didn't much care for World at War. Seemed a bit...trashy. But the bit at the end, where you can kill Nazi zombies? Loved that bit. And as of today, everyone can love it.
Treyarch have "unlocked" the previously endgame-only mode - called Nacht der Untoten - so that everyone can play it, regardless of whether they've completed the singleplayer campaign or not.
It's a smart move, because for all of Left 4 Dead's brilliance, there's something altogether creepier about being locked in a house while zombies pour in through the windows. Nazi zombies.