Great game, good style, awesome design... but
When you pause the game (esc) while reloading your weapon, the weapon keeps reloading while paused. You can then wait for your weapon to fully reload before unpausing.
This is particularly cheap when using the shotgun, as it's big downside (long reload time) is avoided, and you can continuously blast away with it and barely any zombies make it to the barricade (only to be blasted when coming out of pause).
Otherwise, AWESOME GAME!!! The only gave I've reviewed on here.