The game feels unique, despite not being original, and very retro, and while it has some ragequit moments, the challenges are good. It might feel confusing a little at first, but the gameplay feels intuitive. The game is short in a good way, because it doesn't get boring, albeit it does feel more like a teaser for something bigger. I think that this game will be frontpaged soon, and I do wonder if you get some daily prize.
There are some slight bugs, as when sometimes the hit-areas are a tad bit sensitive, and sometimes the player slips from a block to a lava if he doesn't stand "properly" on it, but it's all minor, not even worth deducting any points. So, as to what was bad? Hm.... hard to tell, because the game despite its simplicity feels polished. Maybe other users will be more helpful.
The final boss felt a little weird, as all I had to do was to drop one rock on him, and then I got transported to the next, final level. At first I thought that I died and got transported to previous level.
Hint for players: to get to some of the higher platforms you need to run, which is especially important in the first level, where there is this spot that you need to get to get the last crystal and finish the level. Experiment with running and jumping.
Also, if you press continue before starting the game, you'll go to a bonus room, albeit there isn't much to do there.