The problem with the collision detection is that the mining would be a pain. Because of the ore veins only being in certain places only 1 or 2 people could access it at any one time.

Also getting into areas that people have blocked off, im no master on code but i think that the walls and floors would have the same code for their solidity as that of which players would have. in that case wouldnt you /ghost command make you fall through the floor and be able to walk through floors?
(that was what i think and also another question. please correct that if any of it is incorrect  )