Subject matters.
It is possible to produce nice prints at 22 inch from a 12 mp camera when things are done well.
Looked at by itself the print will feel fine.
Now with subjects featuring textures or small details, like distant leaves, the D800 print does provide a significantly more refined impression, it does feel more real. That's a bit like looking at a DVD vs Bluray on a 46 inch TV. The upscaled DVD is watchable, but it is hard to go back once you have gotten used to the BR image.
Besides, DR and colors with the D800 are nice!
Cheers,
Bernard