On two occasions Nikon has stepped up megapixels and had a lot of complaints.
The D200 was the first major step in megapixels. I think I threw away 100% of my images from the first two weeks. Technique was relatively forgiving on earlier cameras, but the resolution of the D200 caused many users to have to concentrate on technique for sharp images.
The D7000 had a similar uproar. While there may have been some camera issues, the biggest challenge was technique - especially on a camera within reach to consumers. The D7000 has higher resolution and does require more attention to technique - particularly focus, depth of field, and sharpening.
With FX cameras we see the corners, so lens problems surfaced as a major complaint. Nikon quickly had to upgrade several lenses. The wider image caused fewer complaints about image quality - and raves about perceived sharper images.
So expect the D800 to have lots of complaints over soft images. It rewards good technique and penalizes poor technique. But the image size is not very different from the D3x and cropped it is similar to the D7000.