I went trough your demo, I only now can give you a more detailed feedback.
In terms of concepts, ideas and visuals, I'm pleased to see all of what you made so far. The hack sure is unique, going with ideas that haven't been squeezed so much by other hackers. I hope that you'll be implementing more abilities and gimmicks, so that it doesn't get too repetitive.
Now, some things that personally bothered me.
First thing is how you've hidden the secret exit, which, for a first level, it's too much, since the player only now is learning what the protagonist can do. I'm assuming this won't be the real 1-1. If it is, then imo scrap the key and add it in later levels. Perhaps you can use this way of hiding the secret exit path in a later level, when the player has full grasp of the new abilities and jump height.
Speaking of stuff that doesn't fit a 1-1, the black level part defiedly is. Again, I'm hoping this was just a teaser of a very late level, since this was incredibly demanding compared to what you do in the grassy part.
Aesthetic-wise, I also suggest to not use black lines for non-solid object (like the dark brown dirt in foreground). I'd also make it more obvious that white-lined clouds can hurt you in the black section of the level.
Overall, I sure look forward the eventual relase of this, since it's very cute and original. Good luck with its development!