Appreciate the effort you've put into making it so you learn new game mechanics as you complete the different levels instead of just explaining everything upfront. Also having everything bound to keys (I don't remember if you mentioned it but I immediately hit 'R' when I screwed up and dropped a key into the spikes) was really helpful. The keys in the slimes were hard to find - they really looked like mouths at first!
That said the level design was fantastic - the epitome of simple to learn but hard to master. If I had to add anything it would be a level counter; I don't think I would have skipped the last level if I knew I was about to win. Overall excellent game!