Love the game! I'd wish there were more games of this genre! (If anyone knows, please tell me!)
That aside, it can be improved somewhat. First, it's too difficult. And I like difficult games, but not when my default weapons feel useless against many enemies. Also, those enemies that spam missiles are freaking difficult!
Another point I need to make is that the upgrades should go in a special upgrade slot instead of a weapon slot I think. I liked the variation of the spaceships, too bad you don't know what you're getting before buying. The same goes for the special weapons, they are too expensive to try them out, and money is really difficult to come by, except for achievements, which are one time only.
The rest of the game is great, I love the concept, the implementation, the gigantic 2D spaceship war! All in all, 4/5