Screams don't count as notes.
Considering the fact that Left 4 Dead is not really a musical game, and that the few musical moments and clips in it are quite short, I think what you've put together is fairly good. But as a song its not great. It feels like I'm watching a clip of a bunch of different gameplay footage from the game but they took the video away left with only the gun noises and screams.
As I said, for what Left 4 Dead has for sounds its okay, I mean the game only has like 3 or 4 different song loops in it and they're nothing amazing..but this here isn't really anything amazing either.