There should be some goal or the game getting more and more difficult over time. Because I got to 11 minutes (yes, I couldn't stop playing) and the game was basically still the same. I liked a lot the shield enemy and even if I think that the explosion was neat mechanic, I think shield should block it.
Also, I love how many upgrades there were, but by my taste it would be better if the bullet was faster (and slower to rotate) in the start. But I don't know, maybe all that I am saying was in the game, but playtesting showed that it isn't a good idea.
But overall, really good game, I have fun playing it and I couldn't stop.