Very polished, but unfortunately very little game! Looks good, sounds good, feels good (UI feedback and everything). I definitely would have played more :)
That said, the "puzzles" that are present are entirely linear, there is nothing to think about, throw your heart past the obstacle and run to collect it again, then continue your platforming to the key card/door. Given just the introduced mechanics, I also don't think there's much room for interesting puzzles with only the currently introduced mechanics. I understand it's an issue of time/priorities, but something like a conveyor belt or switches would allow for puzzles that require some thinking.
Then there are the options. Out of the box I feel the voice-over is too soft - I didn't even notice there was one until the third text or so! Also the mouse sensitivity at 50% is still higher than what I'm used to, so I don't entirely understand why this lower limit was set.
I think it's an amazing demo for the scope of the jam, it looks good and the narration is funny, but the core gameplay feels mostly absent.