Oh, and a couple more things:
The introduction/item help page would've really really really helped if it was in the jam-ready version, but alas!
Downloaded a newer version mostly to read the tutorial and it was quite helpful and encouraged me to wander around.
The enemies do lack some sort of a reaction to being hit - in my experience, they just absorbed bullets until they dropped dead, which is kind of lacking in terms of satisfying gunplay and player feedback, so it might be worth giving a look.
Also it would be nice if the powerups were a little bit more animated, like, I don't know, floating a bit and having a small patch of light underneath them (like the purple projectiles/orbs). Not sure if it would fit the style, but a just feeling I had.