The black crystals were pretty annoying. It's not apparent what they do at first and even slightly touching them means instant death.
It's not clear at first what your items do and when you experiment with them you have to die to get more. A good puzzle game like portal lets you experiment plenty without having to restart.
It's also not clear what you can use your items on and not. For example, on the 2nd level the thruster works on the rotating platform but not the other orb.
Also the instruction papers can be easy to miss. I didn't see the first one immediately. It doesn't seem to have any text?
Good luck with your game!