I really enjoyed the mechanic of fast falling to increase your jump height. I had a lot of fun with this one!
For me, it feels like there needs to be some work on the responsiveness and visual feed back of the fast fall itself. I think there are some important design considerations around how and when holding down the "down" key applies the fast_fall force. Initiating the fast fall after a jump feels really clean, but when I hit a spring, I feel like I should have release and press the down key again in order to reapply the fast fall. I was kind of frustrated by how I had to perfectly time the release. I think that's what some of the issues other people were having.
Also, it doesn't seem like I can trigger the fast fall when I simply fall off of an edge, which is also pretty annoying, but is maybe more of a bug.
ALSO also, you didn't tell me I could slide?! So yeah, apparently pushing the "down" key while running causes you to slide. That was pretty nifty.
Another thing I noticed: seems like the knock back affect could vary wildly in some cases.
In any case, once I got the hang of it, it was pretty fun, and the music was really good!
edit: Oh! I also wanted to say: if you're planning on adding some polish, I personally think there could be a bit more effects, like wind falling animations, to add some juice to the whole fast fall animation. I noticed the character starts flashing when he starts hitting terminal velocity or something, which was really cool! But yeah, the way it currently is also works well enough and definitely doesn't detract from anything! Just some thoughts.