Much of that is simply BS.
The small pixel size does not limit the usability of the camera, at least not in the sense that there would be a degradation of image quality in any sense with the smaller pixels. The only exception may be exceptionally high ISO.So images from a Nikon D800 will always be better than from Nikon D3X, a Nikon D700 or a Canon 5DIII as long as the same lenses are used
Regarding diffraction, essentially any decent lens gets diffraction limited at f/8. That simply means that the lens is so well corrected that it does not improve with stopping down. Stopping down beyond ideal apperture looses sharpness regardless of the pixel size. But with more pixels you may have more to loose.
Some or even most of the sharpness lost to diffraction can be restored using correct sharpening, so diffraction may not be the evil as it is often seen.
Now, there are advantages with larger formats. There is no substitute for square centimeters. A larger sensor will always have a better potential for microcontrast if lenses at similar quality are used at the same aperture.
If there is a need to stop down for DoF a larger format will need be stopped down more. But even a larger format will be diffraction limited at f/8 (unless you have a lens that is ill designed or badly assembled), and if you need to stop down to f/11 on the D800 you would probably stop down to f/22 on the MFDB.
In general, whatever format you use, the more you have the more you can loose. The great feature of the Nikon is that it has a well working live view, so you can focus exactly, and a very hi resolving sensor at a very attractive price. You can buy more than half a dozen excellent Zeiss lenses for the price of the cheapest MF digital backs, and many of the lenses Nikon has are really excellent, too.
Finally, going from 24 MP to 36 MP is not a giant leap, and the 24 MP we now have on a couple of Sony Cameras and the Nikon D3200 would really correspond to 54 MP on full frame.
I've just bought a D800E and it seems to be a very interesting and exciting camera. I'm still trying to figure out what lenses I'll get and how it fits in with my other cameras. Will it obsolete the Aptus 75 on Mamiya AFD? I have lots of questions and testing to do.
I bought a description to the lens reviews on Digilloyd as part of my research on lenses, there are quite a few tests with the D800 and a few with the D800E.
The conclusion other people I'm talking to, and that I'm coming to is that the fine pixel density of the sensor compromises the usefulness of this camera.
Diffraction limits seem to hit really early with many lenses, with apertures as low as f6.3 in many cases. Few lenses appear to perform best at an aperture as small as f8, and almost everything is soft at f11.
So the D800 looks like its going to be great where low DOF is needed. With very wide lenses, maybe these wide apertures mean the camera is going to be OK, but for normal and longer lenses, shooting subjects where depth of focus is needed looks like its going to be rather hamstrung. A lens like the 50mm f1.4 Nikkor maxes resolution at f4.5 or f5.6. (DPReview samples). These things are not great for a landscape image where one wants the foreground in focus while retaining distant detail.
I can see that a wide angle lens could help, but I prefer compositions that use more normal to short tele focal lengths.
I'll reserve judgement until I have a bit more experience, but it seems rather ironic that a camera that at first seems tailor made for use in landscape photography is irrevocably limited by diffraction softening. Nevertheless I'm sure I can fit this camera into my stable and find a number of good uses for it, it just doesn't seem to quite be the revolution I just thought it was initially.
How are landscape photographers who like shooting with anything other than a wide-angle going to deal with this?