Health: Make it drop more often... every 100 kills?
Hit detection: I got hit by a zombie that was behind me and too far to hit me. Didn't make sense to me.
Powerups: The holy bullet thing was cool, The red bullet thing didnt make sense. It just shot through zombies and made them lose arms and stuff, didn't kill them.
If you're not going to increase health drops, maybe lower the cost of armor.
Good game, good ggraphics, sound gets repetitive. Hope to see more.