Infinite replayability. Counter-Strike and Team Fortress already showed that gamers will be happy playing these Co-Op or Versus style matches online for years upon years after the games release. Left 4 Dead doesn't really relent when it comes to the pace of the game and if it does, if it's quiet… get ready! The emphasis on working as a team online is great so long as you don't get the occasional twit who just wants to ruin your fun.
In the days where gamers have become epic stories being told on grand landscapes it is nice to have one that is pure, stripped down to the bone fun. That's what Left 4 Dead gives you, despite the cheesy title.