Slow Paced
I bought a G36 and lost motivation...
I seriously think the game would be better if you sped it up a little. I don't think you'd even necessarily have to make it harder, just make the reload times for most guns faster and make the playing field bigger, with more zombies. The gameplay would be less monotonous of you added barriers and walls to the playing field, then randomized them before each level. That way, you'd have a different environment to adapt to each time.
You should also add a button that lets people turn the moaning ambiance off so I can listen to music at the same time.