I like how fluent the experience is. It feels like you cared for the micro mechanics; I'd even say that the hitbox is not a pure square. Gravity relates jumps to the size of the level, so it looks you had a lot into account (which results in coherent puzzles that do not feel improvised).
I find the main echo mechanic quite interesting, it gives a lot of room for puzzles and that is shown in a very small space (so I'd say your game has propositional density).
If I had to improve something, I might have put the echo button to be used by the right side on the keyboard, so you don't just play with one hand... But that's a minor thing. Overall, it is a very polished experience in terms of look and feel!