I found L4D to be crap actually. it doesnt feel like a good horror game. the only thing that got me scared a bit were the witches in the game, other than that, its just " shoot everything that moves"
imo the zombies in L4D move too fast and are killed too fast to be scary. I find the kind of monster that moves slower towards you, but just keeps comming much more scary.
Like Nev said, gets repetative very fast, the AI is retarded (havnt plays it multiplayer, but couldnt be bothered after playing a bit of singleplayer.)
there is no story whatshowever, jsut here you have to go.
the game isnt scary, I'm pretty sure they will soon release a scary version "Lego Left 4 dead" or so. (bet the green and black bricks will freak people out.)