I really like the pixel UI and simple music. Having multiple difficulty options is great.
I had a hard time telling if I was actually doing any damage, having a boss health bar would be nice. I couldn't seem to kill the first boss even after shooting it for 3 minutes straight, not sure if I was supposed to be shooting the heart under the tentacles but I couldn't seem to get to it no matter what I tried.
I also like the inclusion of multiple different weapon types!