Hmm I'm not sure. Look at World of Warcraft, it looks OK-ish (by today's standards) but mostly because of the great texturing; poly count is way low. The reason is that the world is just do damn big, and I suspect it's the same with Minecraft. Even with advanced LOD algorithms, the open-ended world concept requires so many polygons to render that today's hardware is simply not sufficient to render it. Processoers and GPUs need to improve 10-fold in performance, and games like these will still easily use all that power. (by that time it may be more efficient to just raytrace them, especially as we'll have dozens of cores in a few years...)

Also, I suspect the Java and GPU points are moot, as I cannot image it doesn't use OpenGL. Now the Jave OpenGL implementation or intermediary or whatever may not be as fast as 'native' calls but I think it's not going to make a magnitude difference.

TL/DR: IMO it's not about the graphics quality but about the sheer amount of it.