A lot of good things. The character's movement, and the animation of the different elements in the game all feel pretty solid.
The puzzle designs were good, with room for a good bit of refinement. I found myself getting soft-locked several times where it felt like I shouldn't have. The last stage was like this, getting stuck multiple times just past the swiveling gate. If you don't have the magnet, you cant jump up to the upper level anymore.
I don't think I got the intended solution for puzzle 5. I quickly hit the final switch, and jumped back on the rising platform as it bounced back down slightly, making it possible for me to go back down. Still not sure how I was supposed to do it, but that didn't feel right.
The vertical semi-solids work as one-way walls... but that's not immediately intuitive. Maybe a different design for the vertical semi-solids is in order. It's also not very apparent that magnets can go through these one-way walls.
In level 1, you can do a sort of "Super Jump" up against the one-way wall, if you stand on the swivel platform next to the one-way wall with the red magnet and jump. There's no practical value in doing it in this level, but I'm just mentioning it for future levels.
Would really like to aim the magnet while walking. if you're concerned that this will make it difficult to stand still and aim on tight platforms, I'd suggest implementing a button to lock the character's movement for the specific instances where it's necessary. I've played games with a similar aiming mechanic, allowing free movement while aiming, until a button is pushed (maybe L or R?), which allows the character to continue aiming while standing still. (maybe you get full throwing range while standing still?) I think this would be worth the trouble, as it would make the more "routine" map traversal MUCH more pleasant, while still allowing for more precision when needed.
I feel like the ceiling pipes that suck up the magnet should be set to a "universal" polarity, so that they enter the pipe regardless of color. Several times I had to sit there for a second to figure out if my magnet was the right color, just to enter a door.
What happens if I throw my magnet into the wrong door pipe? The magnet is gone, and I'm just stuck outside the door, forced to go in, even if I don't want to.
Last small thing. The magnet-dissolving field with the little smiley face stop sign on it took me a while to understand. Maybe instead of a smile face, there could be a more intuitive graphic (a magnet with a crossed-out null symbol over it?)