The only not fun part was the checkpoint that was miles away from boss, please avoid that, it's just frustrating in metroidvanias, specially when the boss kills you like ten time . The most fun parts where the double jump mechanic and stomping enemies, it felt amazing,. I spotted some bugs, and other minor issues, I even recorded videos, if you want me to share, but I read you only wanted to know what was fun nor not.
Edited: a friend just played your game and found that there was a closer checkpoint to the boss. But when I played I ended up using two diferent ones that I think aren't the intended checkpoints. I think it's better just to have a checkpoint right next to the boss to avoid confusion.