Great potential, but could use a bit of reworking.
I played through Norway, and I think it should cover up pretty much needed samples for the review.
Hmm... alright. Let's point out those points:
GOOD:
-Nice arts
-Great music
-Minimum lag, kudos for the programmer!
BAD or NEEDS IMPROVEMENT:
-As many peeps has stated, the mouse aiming control doesn't really fit its name. It should be able to cover 360 degree of direction instead of once every 45 degree.
-Sometimes when there's many zombies on the screen, the collision detection seemed to went crapped. Or rather, you can hardly tell what you are hitting. I'm referring to how there's no "redded" zombies indicating being hit when there's blood splatters everywhere.
-Power-up pickup system is good indeed. I don't think an upgrade system is all that necessary since the gameplay is quite challenging, in no means impossible without upgrades, yeah.
-BUT when the normal gun effectiveness was taken into account, an upgrade system for a few things like, say, Weapon Power/Spread, Health Recovery Speed, and perhaps Score Multiplier Timer upgrade, may be necessary.
-Perhaps the stage tileset needs more variation, and maybe add something like hazards.
MAYBE:
-Add an avatar customization option and online score board... but heck, this is just my personal opinion, just add what you deem necessary :)