interesting ... very interesting ... but the hero and the blocks ander him just dont fit somehow.... it would be awesome if he would jump around on the pieces of the world , buildings and stuff (ruins i mean . just throwing an idea for you ...) love the music , waiting the game . it passed my test :D

I know it's a demo, but it's good already, seems like you could do alot with this.

Smooth animation, good BG music and SFX. Might want to include directions as to what the controls are, and maybe a death animation. Not bad, though, for a demo. :)

Could use some work.

The sky "jumps" awkwardly when I go up sometimes, you can't know the controls unless you experiment with the keyboard, and I am not certain if using "cat" from Minecraft (If it is that song) is legal in a non-Minecraft related creation.

Since you called it a demo, here are the things I think need fixing:
The game loops when you beat all the levels. The level progression is random with no explained story. there's no explanation of controls, I know they're simple but it annoyed me having to use trial and error to work out that z was jump. Some of the platforms extend invisibly beyond the visible block. The backgrounds sometimes move erratically when you jump.