Greatest strength in a challenging game is the reward in completing a stressful task and feeling accomplishment, however, the weakness of "hard" games is that many lack uniqueness, many are platformers, many focus on loose controls, and many sell themselves on difficulty. I'm happy to say this game isn't the completely obvious difficult game. As usual a game doesn't ever meet everyone's standards.
Sliding in games frustrates me, I've always hated games with sliding. But I mean since the puzzles are focused on sliding there's nothing to argue against. The game is designed around the ability to slide, which is fine. A game designed around it's movement system is better than a game that is designed for another style of movement. An issue, however, is you still slide in the air. That's because your controls are loose, which means not only sliding in air but as well as slow control responses.
Now my biggest problem with this game is the ending. "You must get under 50 retries to see the true ending". We play games to see the true ending the first time, not some bogus "get under this to win!" nonsense. It is a method to artificially elongate a game. When you beat Halo on Legendary you got bonus footage at the end of the game, you didn't get the fake ending, and you didn't have to win the game within a limit of lives to see it either. Could you imagine playing Demon Souls and not truly "beating the game" if you died more than 50 times? It's appalling and poor an extremely poor and lazy development decision. Which the levels you recycled and just put ice in were also boring because they were still almost the exact same levels. The ice had very little difference in the matter.
Your difficulty curve was weird. At times it made sense, because if you introduce a new mechanic you have to provide players with a situation to learn that mechanic, one that doesn't get them killed. Yet at times levels would suddenly decline rapidly in difficulty.
Aside from all that, this game had some surprising game ideas. I enjoyed the level designs and I enjoyed how there was much more ingenuity in this game than many others. I recommend making the retry button be spacebar as well as jumping so that if you run out of jumps you can just hit space and restart immediately. You should also change the text scrolling to the arrow keys so players don't accidentally waste a jump and read it again.
So where did you get the idea to make this sort of game? What was it like thinking of different methods to produce challenges? And how was it learning this game's system, what would you recommend to people who are starting like you with flixel framework?