There's so much potential in this rewind mechanic! I didn't get good enough to really see how to really use/abuse it, but it felt like I should be able to really stretch my health or get more hits in with. The combat is excellent, and the art and combat are both solid. I enjoyed the enemy variety as well.
I like DoubleFrap's suggestion to make the rewind required in some way, though I don't know what that would be exactly. Maybe a puzzle-y thing like Braid, where you fall in a pit that's a bit too far to jump, then it teaches you both rewind and double jump and/or dash? Not sure.