I'd like to ask a related question. Let's say that I put a pro-quality prime lens on my D300 (I want the lens to not be a factor here) and take a shot of something. Then I put that same lens on a D800 and frame the shot (by moving my position relative to the subject) so I get the same area that I saw with the D300. Now, take those two images and resize them (without any cropping) to, say 1200 pixels horizontally or print them both at, say 10 inches across. Will I see any difference? If yes, why?
I've seen quite a few D800 images on this and other sites now and frankly, I couldn't tell that they weren't shot with another camera. Of course, I realize there are many other differences between the D300 and D800, but I'm just trying to assess pure image quality (ignoring the ability to crop, etc).