Originally Posted by Mrs Beanbag
Then we need to think about how to handle player modification on sub-block scales.
Also bear in mind that each block in the world needs to be updated at each frame if it's currently doing something (some blocks might, for example, need to fall when unsupported, grass may need to grow on newly placed dirt, monsters need to roam wherever they are and if they affect the landscape around them then that needs to be kept track of... that sort of thing).
Of course, all this could
be faked to save on CPU time, but getting the balance between a believable world that's happening all around you and reducing resources is quite an act.