It was great that the flail can also block bullets. The overall game feel on the flail is great and it's a really neat mechanic to put in a shoot em up.
Definitely more enemy types and level variation would be cool as would audio. A really easy thing to implement but that would be nice is to flash the screen red (or just flash the screen corners red) when you get hit since right now there's no indication of it besides your healthbar decreasing.