Really loved the audio and effort on the voice overs, has quite a gradual ramp up but believe that is what you were going for with the power ups complimenting game play ramp up time.
Think one thing I would of perhaps tried to sneak in is bullets from enemies getting a bit more intelligent / harder. Maybe ramp up the bullet speed as the level progresses or even allow some enemies bullets to start tracking / chasing the player.
But you did all this in 3 hours and it is a level of game play!
I managed to get to the end!