I think this is a bit under rated.
It's a very solid defense game. i love the death "physics" (or at least the many different deaths of the oncoming waves). i have a few comments that aren't totally positive.
- gun shots are repetitive.
this one's tricky to get around, but i thought that you might make the upgrades actually affect the sound of the firearm. for instance, say the player starts with an MP5, but if you upgrade the ammo capacity it becomes a P90, if you then upgrade the accuracy it becomes an FN FAL, you know, that kind of thing just to change it up a bit.
- upgrade the type of airstrike
taking a page from COD4 on this one. the first level could be a simple strafing run, the 2nd level might be close air support from a helicopter, and lastly you get support from a gunship. that said, the airstrike doesn't need to kill everything on the screen all the time, particularly if its an on-station sort of thing, it could just be a temporary backup shooter, if you will.
- more forgiving gameplay
a save point, even if it's every 10 levels would be very much appreciated.
Other than that i can't think of many flaws. i loved the shaking screen when you fire, and i liked that the gun actually gets less accurate (or appears to). the game has a very high level of intensity considering it's simplicity as well.
Good job.