Please understand that i know that the physics of a game makes the game, but i have some suggestions for the player physics.
And also i am no newbie to Voxel-based games.

I find it odd that going up stairs is a real pain if you don't have a ceiling to stop yourself from going too high, but then that same ceiling limits how fast you can go down stairs. For me i think the most annoying thing in the game is that and the next point i will make.
When I want to get from my water source in the next zone and back again, it takes me a long time, I am suggesting make a power that allows you to run when a certain key combo is pressed.

And also i am left handed, so pressing left shift while i use the arrow keys, and use the mouse is impossible, so can you make that key customizable?