This could be potentially really good.
1- Put some sound in it (don't forget the mute button!).
2- Upgrades were a little too slow for the lenght of the game (although they were enough to complete the game)
3- It would be cool to know exactly what the upgrades do (instead of "bullets" you could write damage, not so obvious as you might think)
I know there's more that could be improved, but this is on a righteous path.