So many positive things to say here...
The mechanics have a staggering amount of depth for how intuitive/simple they are. It feels like the built-in levels, all well designed, only start to scratch the surface of what can be done. The cropping/scaling/rotating/etc breaks the natural inclination to view a box as a box, a line as a a line, etc. and to view these basic shapes instead as resources for sculpting other platforms. E.g. a staircase can be constructed from a cropping and moving a quadrant of a square, or from expanding the right half of a square outline. Mind-expanding for sure.
Some ideas for improving the readability of the tools: maybe when using the rotate tool display the center pivot of the selection box so it's clear what pivot we are rotating around. That tripped me up for a bit. Same with the scale tool.
Love how easy it is to make your own custom levels, no level editor needed just make a bmp. brilliant.
Challenge for players: world3 cellinthesky.bmp can be solved with only one of the provided tools ;)