Collision with enemies are wonky. Touching sprites from the side and enemies touching you from above don't kill you.
Collision with environment also disturbs flow. Bumping into a wall while jumping should make me slide upward, rather than stop completely.
Also jumping immediately after landing results in a smaller jump, not sure if that was an intentional thing.
Maybe you can also add some other stuff to interact with: gates, keys, switches, etc. The level designs aren't very different, so it's easy to get bored.
Hope the critique helps! Good luck with the game.