I could never get into L4D. The whole thing seemed completely repetitive, with boring interchangeable guns, characters with only cosmetic differences, linear stupid levels and gameplay that had no variations after the first 15 minutes. I even read in some interview that they had their granmas or some such people testing the game and the earlier versions actually had non-linear levels, but after granma kept complaining about getting lost they changed that. Great, playtest the damn thing on people who will never play a first person shooter for any reason other than BUT GRANMAAAAAAAAA I THINK MY GAME IS REALLY COOOOOOOOL
Some folk say it's the multiplayer with friends that makes it fun. Why, I ask you, couldn't you just play multiplayer with your friends in a game that has characters with actual gameplay differences, lots of different weapons that actually behave in different ways and support different gameplay styles and custom levels of the non-linear variety? You know, like Team Fortress 2?
Also L4D2 seems like a "sequel" in much the same way that X-Com: Terror From The Deep was a sequel. Which is to say WE RESKINNED EVERYTHING SOME THINGS
Also:
Left 4 2 Die
2 Dead 2 Live
Left 5 Dead
2 Live Crew