Good:
- Visuals
- Sound
- Concept
Things to work on:
- One of the gold rules of platformer - make it obvious. If ledge/platform is accessible with character jump height/range - make it obvious. Don't make few units higher platform than max jump height.
- Checkpoints. The long level felt frustrating when you fail and begin from the very bottom/beginning. As long as your game doesn't tag itself as rage game - make it comfortable to play and even fail in the game.