Hi !
Nice game but I'd say this : TOO many levels !
Reduce the number of levels by half but make more interesting levels (like different paths to the Boss)
Also, for upgrade choosing (after Boss) you should have only one validation button and make player choose with by selecting it first (some people got confused, thought the game gave them the wrong upgrade).
I played only Lasers + 2 stars Reinforcement x3 and got bored cleaning levels after levels the same way :/ (plus seeing the looooooong list of levels is kind of discouraging :x)