This was a cool demo! Lots of potential. Here's some nitpicks.
-The aim line and smoke go behind certain elements like the doors or the Thanks For Playing but the magnet goes in front of them. Not sure if that's intentional. Certain parts of the hub the magnet also goes behind, like the green key squares.
-From the end screen there's no natural way out besides hitting escape. It's a demo so I'm not sure that matters.
-When I initially went into aim mode, I couldn't actually hit the switch because it wouldn't let me adjust the aim. I was using arrow keys and then pressing E, but the arrows didn't seem to be adjusting the aiming. I only realized you could when I switched to WASD + mouse, which I didn't get an indication was possible initially because there was nothing indicating directional keys.
-When you re-enter a level, the key is gone, so you can't always tell what the goal was unless you remember. And is there even a way to beat the level again?
-With WASD+mouse controls, the F button is the same finger as D, so it's a difficult finger-hopping move to try and get over to the right on the last level. F, move finger over to D, back to F super quickly, back to D super quickly. Maybe in that config right mouse would work better for polarity switch. Different hand.
-On the first level, when I was standing on top of the ramp on in the middle of the screen facing left toward the blue beam, when I pressed E to drop the magnet, not throw it, the magnet zipped back behind me to underneath the pipe. I thought it might have been "rolling" down the ramp really fast, but hovering above it, but I'm not sure what it was. When I loaded back into the level later it didn't happen again.
-If you go back and forth quickly in the flip doors that don't use the magnet, you can get in a position where you're pressing on the door, but you have to go backwards and forwards again to push it open. Not sure if that's meant to be a situation where you need momentum to open the door, but it feels a little off to get blocked like that.
-That flip door also moves slightly before you touch it in certain scenarios. It works just fine visually in most scenarios, but if you're moving back and forth it feels like there's a buffer of air between you and the door that you're pushing on.
-The tail while you're moving is really nice for game feel and conveying a sense of momentum, but it sticks out a little bit amongst all this metal and machinery. Maybe an exhaust cloud would diagetically convey the same thing? Or it would look like the robot is farting all the time.
-It's an interesting touch to make it so the wheel rolls you off corners. Very distinctive and cute. Maybe since you're not going in a hardcore platformer direction anymore it won't matter, but a potential downside under that scenario is I wonder if it's possible to just barely miss a jump and then roll off.
-Pressing left and right at the same time instantly makes you not move and lose all momentum. When hopping back and forth, frolicking almost, it can lead to this awkward grinding to a halt feeling if you happen to still have one direction pressed when you move to another, or queue up your next direction without letting go of the last. You either freeze or freeze in midair and drop straight down. The latter could be a strategy (albeit a physics-breaking one) for more intense platforming, a way to make a huge leap and then just immediately drop with no momentum, but I think it's too easy to accidentally do.
-When you go back to the hub world it would be nice if you came back in the same location that you left from. Outside of the door of the level you just left, for instance.
-I like the solution to level 1 a lot. It did take me a long time because I was familiarizing myself with the momentum of the magnet, figuring out that aforementioned glitch, and a little unclear on how the bar that blocked me worked. Because of the little ramp going left I initially thought I had to get the magnet to drop onto it so that it would roll left onto the other ramp and then roll right before I realized that I just throw it in when it's the other color. It's doable, but a bit of a hardcore first level because there are a lot of new elements to figure out.
-I like how the trick with Levels 2 and 3 really build on Level 1, about changing the polarity when you're far away
-It would be nice if each level had some unique identifier in the hub world. A name, a symbol, because it's hard to remember which is which if you're going back to them.
-The parallax is slick in the hub world, but I initially thought that the tiled background was where the garage doors were attached. So without them being attached to the background it feels like you and the doors to nowhere are floating on a plane half a mile away from the background.
-In level 4 I initally tried to use the magnet to pull the large moving piece to the right because it had a plus on it and it looked like it fit right in that alleyway. I would think that it's just because it's "too big" but the blue magnet doesn't even stick to it, so there's an inconsistency in how the magnet behaves with different magnetized elements.
-In level 4 (it seems) you can soft lock the level by destorying the magnet while the flip door on the right is set to blue, which is really easy to do. So the magnet respawns between the destruction field and the back of the door, which isn't magnetized, then there's no way to get the magnet to where the door can be opened agian. Soft locks I guess are what the reset button is for, but in more complex levels or play scenarios where you're exploring new mechanics and how they interact, it might be hard to know if you're *actually* soft locked or not and just run your head into a wall for a while until realizing that there's no other way. The other major soft lock in this level is if you throw the magnet onto the switch from the alley and then get blocked in. That one's more obvious, if frustrating.
-I like the solution to level 6. I will say when I entered the level I got overwhelmed at the number of mechanics in front of me and just started messing around, because it wasn't clear how they all interacted with each other. If I were playing a fuller game where I had more time with these mechanics I'm sure I would have been less overwhelmed. It introduces an interesting mechanic, blocking the motion of magnetized blocks, that I'm excited to see developed.