Easily Improved...
ed the game... To a degree. Yet instead of simply slapping a bad star rating on you, I'll be kind and give you 3933 or so characters of improvement tips. :>
Frankly, I think that you should make a game called "Asteroid Revenge Enhanced Enhanced" (no typo there). This game wasn't completely terrible. You got your point across and I'm sure that you're happy with the immediate result. That said, this game could have been classified as good to excellent with even just a few improvements, not to mention a complete makeover.
Firstly, the graphical presentation wasn't wonderful. It was simple, yes, but there is a difference between "lazy simple" and "remarkable simple". Your current presentation counts as lazy simple. You could improve upon this "simple status" by including great, modern features such as a general UI (showing how far into the game you are, the amount of health (larger than it is now!), your score) or a nicer menu system (heck! You didn't even have an in-game menu to my knowledge!).
The in-game graphics could be greatly improved without actually doing much. The lines could be bolder and the enemies could be slightly more detailed: the graphics can be retro-style while still being respectable. For example, when a ship "explodes" it could have a much nicer animation. Is that harder to do? No, it's about three minutes work. It doesn't make sense that the fragments of a ship just disappear.
Another thing that really, really bothered me (and pretty much ruined the game for me) was the AI of the enemies. Whenever I exited the screen, the enemies would instantly turn to face me. 'That isn't right!' I thought to myself... The enemies should gradually turn to face you. This would give the player the momentarily gain the upper hand and would give you (as the creater) the ability to make harder enemies. Afterall, you wouldn't want us getting too much of an advantage...
Something that would be wonderful that would incorporate both technical and graphical improvements would be if the asteroid you control swung around a bit more (the controls would feel sluggish but realistic (you obviously can't turn off the dime while piloting an asteroid)) and the other improvement would be if the asteroid itself moved instead of being a static piece of rock that doesn't move at all.
We then get on to the creature comforts (geez.. I'm pretty sure that most normal people won't have read this far but I hope that you do, Mr. Creator)... Things such as a nicer powerup system (more than just invincibility and speed - perhaps larger bullets or all the enemies are weaker for a short amount of time.) or a transition between the levels. Sure, it removes the complete retro feeling of the game but hey, let's face it, the game needs the overall improvement (besides, with something as simple as that change, especially one that doesn't effect the game play, it will hardly change the retro feeling).
Overall, the game was a pretty lame attempt at recreating a classic (with a twist). With even just a little bit of extra effort put into this you could have a nice game in your hands. Also, sorry for the long review but I figure that it's my duty to try and help out.