It's funny because at one stage, most D700/D3 owners had this arguments saying that 12.1MP is way more than enough for most applications, in defense of Nikon's inferior resolution compared to Canon.
This argument holds a lot of truth, who among us are into A0 printing on a regular basis? Some will actually tell you that it is possible to do A0 prints with 12MP if done properly.
And at the time ISO performance were more on the cards, which implied that you can't have too much pixel density.
Now it seems, Nikon fans are changing their arguments.
Fortunately, technology is changing at a rapid pace. In terms of pixel density, we all know that the D800 will have about the same as the D7000 which is already quite a good ISO performer, very comparable to the D700. With 36MP, your cropping ability is massive.
Most important: the D800 will probably have outstanding video capabilities. Something that was seriously missing on the D700 and made many Nikonians switch to Canon.
I agree with bobmielke, there's no replacement for displacement - as long as the rest of the technology follows. If you develop a motorcycle engine that is capable of doing 200hp, the chassis must be up for the job. Same with cameras, if you managed to stack 36MP in a sensor, ISO performance should not be affected.
Stangman98, I think your concerns are answered by the D4, but at double the price of a D800. The D700 is not being discontinued by the way. So if video is not a requirement, the D700 is still an excellent camera.
For me, the D800 ticks all the boxes even if I won't really need 36MP. My main concern with the D800 is storage, image files are going to be quite substantial. Fortunately, storage devices are becoming bigger and cheaper. So it's actually not really a problem!
