I liked the inversion of the typical power-up mechanic: picking up souls and becoming weaker is surprisingly fun. The final segment didn't quite stick to that (I'd have loved to see it take away something else, like moving on the ground or something), but it was more challenging than any other area, so it still worked.
One thing that was surprising was that the first challenge (the double-jump) was a lot harder for me than any other. I think this was due to the jump being based on key release rather than key press: getting the second jump to trigger at the peak of the first meant I had to release the space and re-press it before the key had fully risen (otherwise the time between press+release was too long).
Since your comments sound like you'll be continuing to develop this further, I wish you the best of luck with it!