It's hard to see what you can walk through and what blocks, that makes it lose quite a bit of wanting to play, because there is little space to dodge enemies sometimes, and while exploring you come into areas, where you don't know what will block you when the enemies come.
When not playing full screen the powerup selection, didn't scale (they were out of vision and way too big).
I liked how fast you get upgrades and that they help meaningful (crow ally for example)