There may be an issue with difficulty progression with the puzzles due to the environment being a large nonlinear puzzle to solve -- really it is a set of linear puzzles to solve, just gets confusing as to what order to do them in. Basically the first puzzles are easy to solve and then it ramps up steeply.
FYI I don't normally play these kind of games, I have very little patience for them and get very little enjoyment out of them; so I didn't play all the way through to the end once I was stumped by my way forward being too unclear as to what to do next -- I made it to Area 3, accidently fell into an area that I could not jump out of; no way to reset.
Too many puzzles available without it being clear to me which needs solving first in order to progress further. That may be a me thing or you just have the difficulty ramping up too quickly. As far as content for a demo, I'd say you have that but you need to work on pacing as you are presented with too many puzzles to solve in order to determine their sequence of solution needed to progress.
As far as presentation, generally the visuals are fine. I would experiment with post-process effects like bloom and such in order to elevate the visuals without making the scene harder to read. A lack of sound is a hindrance as it fails to set a mood for the game or draw you in immersively -- simple set of sounds for footsteps, block moves, puzzle control interaction, ambient environment sound or music, would make the experience more enjoyable.