Pleasant background music and soundeffects. 

Straightforward controls, easy to figure out intuitively.

Was confused at first about the bottom 2 buttons to buy things, but figured it out so that was fine.

Small bug (haha): after buying health once, I think I became immortal. The health text vanished, and even when I left the game running, I didn't reach game over.

Other than that, game felt very clean and complete!