I love this game.
1) Lack of a story / Plot.
2) Only 4 playable levels via the single player and online co-op. (Minus DLC or GOTH version).
3) Single player is almost impossible to play on the higher difficulty levels.
I write the negatives of this game first to make my next point stand out even more.
This is one of the best games I have played in a long time.
This game proves that less is in fact more. Valve has taken a handful of good ideas, and implemented them to near perfection. The result is a game that never lets up on you and never lets you down. This is mainly due to the games fantastic spawning system. You could play this game all the way through 100 times, and every time feels like a new experience.
At a few good friends into this mix and you will find a new addiction that no intervention could ever cure. Teamwork is needed to succeed in this game. If you go all "Leroy Jenkins" into a level you will not survive. Jack up the difficulty and the tension becomes unreal.
In conclusion this game is FUN! I can't wait L4D2.