Beat the game and got to the bonus level where it said "they just let anyone design these rooms huh?" before stopping.
Good game, great concept. Very appreciative of the Linux build!
In my opinion though, it overstays its welcome a bit, and a lot of the levels felt same-y.
The game has a bit of a sporadic difficulty from where I'm coming from. It generally increases as the game goes on, and carefully placed decreases in difficulty can provide some relief from the action, but I feel like it fluctuates a bit too much (not counting the bonus levels, just the main game). The big setpiece level with the big rectangle with gears is a huge spike in difficulty and definitely should've been closer to the end than it ended up being.
The game is also noticeably glitchy. I get that this is the nature of physics-based games, but definitely something to look into.
Something I liked about the game was the degrees of freedom. Each level had a planned solution, but most offered room for more wacky solutions. This decreased the closer it got to the end, and I'd definitely look into maintaining that freedom, while also having a steady difficulty progression (not easy, I know).
You seem to have plans to make this a priced Steam game, if I'm reading between the lines correctly. This implies that there will be more levels and content overall. You might already be doing this, but I'd add some new objects and hazards to spice things up and reduce the monotony present in the demo. Perhaps a block that, when punched, spews shrapnel parallel to the angle that it was punched at, or one use items that rocket you really far when punched, (perhaps even boss fights?!?!?!?! Maybe not that, but a guy can dream...) stuff like that.
This may sound critical (and it is), but I only say all of this because I really enjoyed what I played, and want to see an eventual priced release be as good as can be. The game currently sits in my Steam wishlists and I look forward to the full release.