I don't normally take the time to write reviews for stuff like this, but to be fair, this is probably the one zombie game I felt genuinely entertained by!
First off, the weapons felt balanced. And each had a good quirk I felt. The thing with the pistol, for example, where when the zombie was knocked down, was nice in indicating that it was down for good. But I didn't think knockback was implemented until I used the machine gun. It's a nice twist! Admittedly, at first it was an annoying feature, but then it gave me a few surprises when you blindly go around with the minigun, and happen to miss a few.
Also, the feature of bosses is nice, and the incentive to take them out carefully is well done. (KA, contact = insta-kill) The missions were a bit so-so however. Other little scattered collectibles though added a nice variety in gameplay.
Though, if I were to chance anything, I would try to see if you could do any kind of variance with weapon effectiveness. AKA, certain weapons work better against certain enemies. Near the end game, despite having all the weapons, I found myself simply using the minigun for everything. the reloading time is a good balance, but the other weapons didn't seem to cut it very often. And the shotgun becomes ineffective against bosses, and armed zombies.
All in all though, with how much was going on the screen, and the overall presentation of the game, I was sincerely impressed by the level of quality and fluidity of it! (And my laptop isn't exactly up to Gaming specs either.) Wonderful job! I highly recommend this game!