It seems like you have a pretty good start for a game here, but it could probably use a bit more work/polish. The armor and ammo HUD take up a pretty significant size of the screen and could be reduced by quite a bit. Being able to hold down the mouse button in order to shoot automatically would be a much appreciated addition - having to keep clicking in order to rapid fire can get kind of annoying. It might also be a good idea to have the bullets be circular as well, considering that the current bullets don't seem to rotate depending on where you fire from. Having some sort of explosion effect for when enemies are destroyed would also probably make the combat seem a bit more interesting.
Overall, I think just putting more time into this would definitely help it out a lot.