Cool! I have a suggestion though...
One thing I've noticed was that I got all hyped up into playing the game in an instant... like... wham! Simple as it may be, its ability to immediately deliver the gore factor I was looking for produced the game's longevity factor. In other words I think it keeps people into the game for as long as it lasts.
Butchering (or getting butchered by) the zombies by bulk was a great idea, and it was rather stimulating. I think it was a major factor and perhaps the very life of the game.
Thus... I suggest adding a panic button. Something which could be used like once or twice when all those zombies manage to completely surround you. Like maybe perhaps the character uses a grenade... imagine the gore factor of that! Or he uses a chainsaw or some sorta blade weapon to hack everything beside him. Imagine the adrenaline rush! All that with a push of a panic button, perhaps with a right-click or the space bar.
Just a suggestion =) Great game!