I've been a big fan of your YouTube series, and I'm very glad to see you making your own game!
I don't think I have much to add that wasn't covered in your postmortem video; I agree with most of the feedback. I was particularly bothered by the magnet not being solid when it was on the floor. One thing you didn't mention that bothered me a little is that touching a door automatically forwards you to the next level instead of requiring a button press. I'm just primed to think I have to make characters walk through doors, I guess.
The awkward fusion of action and puzzling was definitely a problem for me. That is something that bothers me a lot in puzzle games, as it's frustrating to not know whether I'm executing the process wrong or if my setup is wrong. I think part of this may also have been the issue with the character controls; I like platform characters that are fast and fluid, with low friction and high acceleration, but this character was very slow and cumbersome, so movement wasn't as fun.
My favorite levels were... uh, I don't remember the names (it might be a good idea to clearly display level names when they start, since it makes it easier for players to give feedback), but the one with the three magnets that could lift the magnet but not the character, and the final one with the moving plug over the spike pit. I found the solutions quite a clever use of the game mechanics.
I did run into an odd bug in the room with the two horizontal lasers, where I threw the magnet above the lower laser when it was in its lower position, and it landed on top of the laser, as if it were solid. Moving the laser eventually made it behave correctly, but that was strange.