This is really a great game. It is easy to play, simple, and has a very addictive element to it. The music is good and I enjoy the random upgrade system. Making the game play shift and adds a value of re-playability, which is something rarely seen in a flash game. I personally can't see any real way to make this game better, outside of adding some more tank customization options though that really isn't going to lower my score any.
I think something that would be cool is a map editor. As a game like this could get a lot more interactive if you allowed that and maybe make a place for users to upload these maps for others to play to. Again great game and keep up the good work.
-murderer5