The first thing I noticed is that I and the enemy are constantly shooting. It's weird because if you wait just enough for the bullet trail to hit the bottom of the map it's impossible to hit an enemy without running into the trail of bullets first. .. Is that intentional? Seems as if later down the level chain it would be almost impossible to survive with that.
The explosions are awkwardly slow. I like the graphic that occurs during the explosion (money-burst and all)- but the orange cloud disappears too slowly and gives an illusion of lag.
No respawning on death? Could just put a gameover with a button that restarts the layout for now.
The upgrading is insane. The first star makes me feel ultra powerful and anything past that is too much. An idea could be turning off the player's ship's auto-shooting. Then maybe put a cooldown and allow upgrades to lower the cooldown, higher the damage and even increase the amount of bullets at a point.