Great polish, lil bit more content

Definably an awesome achievement, great game with obvious effort put into the polish and refinement. But it seems incomplete some how. There is now doubt that the graphics are amazing for GM game, not to mention the speed and polish applied to the interface. On a technical level, this is indeed an award worthy game.
The short comeings how ever are of the time that really bite game designers in the ass if you don't pay attention. Here's a list of things to try on it:
1) needs more varied music - the sound effects are awesomely balanced to the background music, but its the same background music. Mix it up!
2) no penalties for dieing - I loved playing it, but after dieing a few times, i noticed that it merely became a minor inconvenience, you can correct this simply by adding some "death" sound effects, and removing say 5000 to half of the credits whenever you die. Because the money stayed as is, i was able to buy all the upgrades fairly quickly. This removes a lot of challenge from game-play.
3) Needs to stop being so straight foreward - its nice and good to play it but it still plays like a demo/proof-of-concept rather than fully fleshed out game. Maybe a splash screen, a storyline, even some cheap cut-scenes that depict an alien invasion. doesn't have to be a good story, just something to tie it all together.
4) to damn short - when i beat a game in just 15 minutes, i feel ripped out of a game experience. It needs more levels and bosses to go with those levels. you have essential one level with 3 stages that repeats. Maybe you can make 3 levels. One for defending the earth, second for traveling to the alien homeworld, third for striking back at those aliens. I'm not saying to stack on hours of game play for the sake of lengthening it alone. Nothing wrong with a short game. Just make the levels varied and you can extend the game without killing the quality you obviously worked hard on.
5) Do easy to get rich - This is a minor balance issue that can be solved by lowering the amount of money you get for each kill, and/or raising the price of the upgrades. I was maxed out well before the last stage. Then it became cake.

It may sound like i'm ripping you a new one on a simple game, and i'm sorry, it really and truely is an awesome example of the power of GM, Its just that we all know it can be much better. With a few tweaks,(i say few, but i know those are pretty hard tweaks) I say you make a sequel that takes this game and does those expansions on it. Since you got the hard part of it done, (the general engine) you just need to build up the rest. Keep developing this game, it is, and will be, awesome.