This doesn't feel like a prototype, this is a fully fleshed out game. Everything blends together so well, the artstyle, the environment, the animations, the music and sfx, the abilities, the level design. The little ingredient at the unreachable spot to hint that there will be a way to get there later, the fact that rooms for backtracking play both ways with the newly acquired ability, everything is SO well done.
The only thing hinting at the fact it's a prototype is the pacing, you get both the upgrades fairly quickly, in a full game that would probably take longer. Other than that? Just wow, I'm out of words, I can't pinpoint to a single thing that I disliked about this.
Ok maybe make the respawn timer a teeny tiny bit shorter..