I really like the artstyle, and the impact the weapons feel they have. I also like the music it gives it a bit of tension. The story is a neat intro to the game as well.
For me personally knowing that the lvl up system was a last minute add it worked pretty decently. I would switch the colors so it fits more with the game and I like the extra weapons more than the just powerups as they change the playstyle a bit more. But as a last minute add, it did add quite some extra value to the prototype for sure. :D