I start with the basic assumption that each new generation is better than the camera it replaces. Thus, I assume the D4 to be better than the D3s (the last benchmark for the D series) and I expect the D800 to be better than the D700 and I expect the D400 to be better than the D300s (that last version of the D_00 DX sensor series). Better in what way? Better resolution, better high ISO clean image and better dynamic range. I expect Nikon engineers to work on all these items. How much better in each of these aspects? That I don't know. At least some of the improvement should be easily visible in comparison photos.
Now it could be that Nikon changes the focus of a camera model. The D700 was designed to be a reduced version of the D3 and the D3x was designed to be the high resolution version of the D3. But now the D800 may be the high resolution version of the D4. So the D800 may be more a replacement for the D3x than just a replacement for the D700.
It will be an interesting year and of course we won't have any solid basis to judge any of this until we have production models of each camera independently reviewed by many sources.