I have spent hours playing this game, trying to max out my completion, and every second has been very enjoyable. It is good at going from easy to hard, not too fast and not too slowly. This game is really fun, has a great soundtrack, and is overall just awesome.
One thing, however, is the story. There is not much of one, and if the creator incorporated more of one that would be great. Maybe even a boss fight every 15 worlds, I dunno. Think about it.
Some features you could add into upcoming worlds are: jerboa Mech suit, shadow jerboa, more jerboas, different enemy types, multi-room levels, I dunno.