Consistent aesthetics, clear sound design, intuitive gameplay.
A minor issue: when playing fullscreen, since everything is bnw it can be hard to understand where is an in-game void and where is the outside of drawable area. This makes it hard to understand where you can go through the side to the next xcreen and where you will just fall.
Suggested mitigation: detect fullscreen and draw a white frame.
Nitpicking: battle system feels completely detached from the non-battle system, but that would not be such a problem if the game were longer to build up more creative connections between the 2 (undertale style).