Though the Nikon D800 was arguably already the best camera on the market from an image-quality standpoint, the natural progression of things means that the D810 has been tweaked somewhat. The most significant improvements come for videographers with the lower base ISO.... but that won't stop traditionalists from asking: "How is the high ISO image quality?"
It's fine, thank you very much.
Aug 2014 Update: Nikon has now issued a service advisory for bright pixels accumulating during long exposures. See the follow-up below for details.