I think the aim/fire mechanic would be way more fluid if you separated aiming and firing (as in, RMB aims, and releasing it cancels the shot, and LMB fires). It would be a lot more intuitive to me because a lot of games work that way. Maybe changing polarity could be done with the mouse wheel or MMB?
Also, I thought needing to throw the magnet upwards to enter a level was clunky and cumbersome. I would much rather it just be a simple door.
I liked that there was a grace period for jumping after leaving a platform, but I feel like there should be one for jumping onto platforms as well.
You should definitely be able to aim while moving. I know it'd break some puzzles, but I think it would add a lot of depth.
PS: That was cheeky at the "bonus" level.