I would hardly categorize WoW or any other MMO for that matter to be in the same genre of multplayer games (as in a multiplayer game that doesnt require a daily subscription to play). It is a multiplayer game per say but not that to what I was comparing my argument too (Unreal Tournament, Counter Strike, Team Fortress 2 etc) and L4D came out on November 18, 2008. CoD4 was released November 16th of 2007 and has sold 10 million copies as of June of 2008. My statement was "since" UT2004. Now after CoD4, we could make the argument for L4D over CoD4 but I dont think L4D has reached the sales CoD has yet (and it hasn't really been released long enough to make those kinds of numbers). Anyways, I'm just saying it's one of the more popular games being played today on multiplayer which is why it shouldnt be changed from it's original game style / roots which it was created.
I do play WoW and own L4D but dont get into either so maybe I'm out of the norm and just dont care for those games as much as other gamers. But then again, I love CoD4 and I know your not big into it so we all have our preferences when it comes to games (i hate UT3 but you guys seem to love it. Though I was in love with 2004 for over 2 years after it's release)