I've always like the pictures from my D700. There is nothing objective or scientific about it, it's just my beliefs.
I usually shoot with my D810 and D700. The D700 seems to give me more accurate focused pictures. The shutter and mirror sound is loud and oddly reassuring. Despite or because of its 12MP, I really don't know, I think the pictures always look sharp and clear. It has that clarity. I am not saying the D810 is bad, actually when I shoot with the 16-35 f/4 with flash and pan I got some very solid pictures. I actually find this is my sweet combination. Still there is something about the overall clarity of the pictures from the D700, the color is not as rich or deep as the D810 but in most situations I find them nice or nice enough.
Fast forward to D4s, I haven't bothered to do any test except one if you can even call it a test. And it troubles me that the pictures don't seem as sharp as the D700 ... though it's only based on one single event. I even tried to do some test, like with tripod and strobe with fixed shutter speed and f-stop between the cameras. The result is virtually identical between the D700 and D4s. Alas, I was shooting at 1/250, f/16, ISO 400 with qflash at manual output at 1/4. I guess I should have done a set at f/2.8. Sometimes when I shoot cycling event and the D700 returns sharp pictures, and at the line up I can see the pores of the face. I haven't got the chance to shoot with the D4s on events like that. I hope the camera can return same or better results than the D700. It's hard to believe if it doesn't but then you never really know I remember reading some online (!) review saying the D4s images are soft and have Nikon USA checked out the camera. I don't want to be that buyer. I don't think it's the AF as I feel the AF is accurate. I've never done any AF fine tune with any of my cameras, not before not now. Maybe it's the lighting condition. When there is not enough light or contrast it's hard for pictures to look sharp. I will find out from real life shooting soon enough. This kind of first world problem is annoying and embarrassing. When I tell people this kind of problem I imagine people would just roll their eyes.