Thanks for the feedback! Yeah, other people have also disliked up being used that way, in the long term I definitely want to make inputs rebindable, but I'll still have to think about sensible defaults.
The physics definitely need some getting used to, but I think they make sense. The whole thing is based on inertia. To simulate that, the game applies the inverse momentum change of the world block to all stone blocks and crates every frame, which means if you are accelerating with the world in hand, a force will be applied to the physics bodies. Usually the friction between ground and the body will be too high and prevent the bodies from moving. However, if you are also falling down, an upwards force will be applied to the bodies (sort of similar to how you can simulate zero gravity in a falling plane), reducing the friction enough for the bodies to be able to move. I hope that makes sense?