Like the Last Stand, but not as good
Don't get me wrong, it's not a BAD game, but when you're making zombie defense games, you're up against a lot of really great titles. If you're looking for suggestions, get rid of the movement keys altogether- they're really unnecessary, unless you make the area in which you can move a little bigger. Even then, moving the character really doesn't do much for the game (sure, you can hit two zombies with the same bullet from some angles, but this game's not really deep enough to require that kind of strategy). The "HEALTH" dynamic text is selectable, which changes the cursor from crosshairs to an insertion bar, and makes it harder to click. Easy fix. I'd also recommend changing up the graphics and sound for different guns. I thought I had the shotgun at one point (I actually bought the SMG instead), and I was wondering why it was shooting all rapid-fire like. An image and sound of a SMG would have let me realize my mistake a little more quickly.
It's all about time, though: it's obvious that you're a capable game designer, it's just a matter of how much time you're trying to put into it- I know we'd all like to see more variety, in every aspect of the game, but I'm sure you've got plenty of your own ideas for that.