I'm really curious (while i don't know really)
What D800 leaves out? What should it have,
to be better than 5D III ?
I don't think it's too much what we're asking
As soon as you shoot off 1500 photos at a event, and bring them back to post process, you will find that the time to process each image is about 5-10 times longer. That runs up serious amounts of time that could be earning $$$. Then, at high ISO's, there is a ton of noise. If you downsize the image to 8mp like DXO does, then its pretty much invisible, so go buy a 8mp body instead.
Next, is the live view, which I likke to use extensively. Its so slow that its very difficult to use for manual focus due to the delay.
Finally, Nikon is missing lenses that equal the 24-105mmL, the 135mmL, and either the 100-400mmL or the 400mm f/5.6L. I used a 24-70mm f/2.8G, and it had so much CA that lightroom could not correct it at the edges.
My fingertips were also always touching the buttons on the rightside by the grip (I have long fingers), this locked up the shutter button.
At ISO 100 - ISO 400, the camera was supurb, but I tend to use ISO 6400 and up, where the noise causes monster sized image files and a looooong time to run NR.
The D800 is a very nice camera for landscape hobbyists who do not shoot thousands of images, and can spend hours setting up and taking images and then in post production to get that one perfect image. You do need a heavy and stable tripod as well as a top lens to actually get the benefit of those 36 mp. Camera testers spend hours trying to beef up their already excellent techniques and setups to be able to see the improvement.