It's a nice little game that does what it does very well. However it doesn't do all that much, and in my opinion not enough to make it a decent seller.
The main things I would consider changing are:
Stop the camera from rotating with your ship, gave me a headache in just a couple of minutes of play time. Either make it fixed or have it rotate slowly towards your current angle, rather than being directly attached to it.
Make it easier.
Better graphics. I know you are going for a retro feel, but this can be done and still look better than this. In my opinion Princec's (Puppy Games) Ultratron does this style perfectly, take a look at his stuff if you haven't already for ideas. Fighting green, flatshaded triangles just wasn't as fun as killing something that actually looks like it might be hostile.
Overall I did like it, and had a fair bit of fun playing. Good job.