The main issue to me is that the mark 3 is takes better pictures than the mark 2. I find the low ISO pictures are fine. If you comparing the high ISO 25600 shots to the mark 2 they are much better and the noise is more pleasant. The d800 25600 noise looks bad from what I have seen. Look at the final picture on this site:
The noise has a lot of random color in it like the mark 2 at high ISO. I think it will be nice to have more wiggle room at high ISO myself.
Up to ISO 6400, the 5DIII and D800 perform very closely. I was expecting The 5DIII to have a clear advantage based on the early jpeg samples, but in looking at the raw files, the D800 certainly doesn't embarrass itself. The 5DIII clearly pulls ahead by 12,800 and 25,600. The D800 looks terrible at 25,600, but I can't think of many situations where I'd need to shoot above ISO 12,800. That's why the D800's supposed advantage in DR is of particular interest to me. I say supposed, because I need to see more images from both bodies before drawing a final conclusion.