WOW! I'm impressed.

I like some of the small details that you put into this game like different gun and explosion sounds. Especially I like the independence animation between same typed aliens.

I also like the cartoonish graphic style you used. Now the only thing I would recommend is making background (space much darker). Blue color doesn't really fit for space background.

Few of my quick suggestions for game improvement at this point would be:
- player ship upgrades either through ingame powerups or by ability to buy bigger and better weapons between the missions
- weapon projectiles that can also fly in diagonal directions which would make game a bit more interesting
- you could improve (debriefing) narration by showing its pictures in sequence (show first, then add second, etc.) like you can see in many electronic comic books.