The most notable absences in this game are audio and fullscreen. You say it's an early prototype but it feels really good even in its current state. Art style is beautiful, some neat easter eggs in the background, and the core mechanics just feel solid. Depending how well focused I am, I'm generally going somewhere from 11 to just over 50 kills (my record is 55 so far). Definitely feels like there's room for improvement on my part.
Also, I'd very much like to see control remapping options. Being able to move the buttons around (and view them in-game, not just on this page) would be much appreciated.