Good game, and simple game play.
But you should:
- Change the sound effect when you shoot (it sounds like a person getting S&M treatment, the whipping sound etc)
- Add moving background, so the player gets the feeling that the spaceship is flying
- Have explosions when you hit the enemies,
well when they die too so the player feels like he or she is actually killing something.
- Add some A.I, so it looks more realistic to the player.
Other then that, good game.