Let's see. I really liked this game: you managed to create what felt like a good progression in difficulty, which is not easy to pull off, and a coherent mood too, by using assets from different sources. This too is no small feat. I also appreciated the simple but clever humor with the title and opening narration.
The balance of challenge and fun is, to me at least, undoubtedly the strongest point of this game, and it is what kept me playing for quite a while, trying time after time to top my highscore. It simply felt fun to play and play again.
Personally I did not mind it being a little more on the slower/tactical side rather than pure fast paced action, as a matter of fact I think it complements the "crowd control" gameplay well, but that might be just me. The fact you cannot shoot through bonuses on the floor is actually a plus to me, for it forces me to think and actually be smart about my positioning.
There is however room for improvement, at least in some regards: the balance of the game, as you yourself said, needs some tweaking.
During my playthrough for instance, it felt really easy to create a snowball situation with the powered up shot bonus, to the point where I could sometimes clear a dozen or more enemies in one volley of bullets, pick up the power up again and repeat, while pretty much ignoring the whole overheat mechanic. My rate of fire was so high, even multiple bosses at the same time could do very little to stop me.
The enemies themselves might need some reworking: I almost never felt like those who could not shoot posed a serious threat. I was able to almost literally run circles around them without getting hit, and that was before I realized I could snag them into objects and keep them running against walls. Enemies who can shoot on the other hand pose a real threat, and feel fair to me.
I have seen others suggesting to add some invincibility frames after taking damage so that your entire healthbar does not get drained too quickly at the first mistake, and while it did not bother me too much, I took it as part of the challenge, I can see their point, and I believe it is worth taking into consideration.
These blemishes are however minor, especially considering you only had 72 hours to work with. The only real shortcoming could be seen in the "innovation" category, but there is nothing wrong, I think, in choosing to try and create a solid consistent and cohesive game over a crazy innovative one, and a solid game this is. All in all, excellent work!