Loading random generated low-poly dungeons etc. in an engine which was optimized for the machines of those days vs. Minecraft's still inefficient voxel chunk loading... Sure, "they" included some better ways to do things (like the map save format which came from the community, the OpenGL culling etc.) during the last year or so but Java as well as "voxels" playing the main resource hogs here. Ok, not voxels in the true sense, but no polygons either.

Or why do you think that the height limit of Minecraft maps is still 128 blocks? A greater limit is not introduced from the fact that world updates would then be a**-slow in the current implementation / engine.