As simplistic as the graphics are, you made the most of them thanks to some good effects and a simple color palette. World was condensed enough to always have the next thing ready to go and the contained encounters were designed well.
I did find the gun charging mechanic to be a bit unintuitive, though, and dying and going back to the very beginning was brutal, especially with enemies that killed you in one hit.
Overall, though, I had a fun time with it!