as an engineer, kinda a silly rant, of course, designers try to reuse as much as possible, otherwise the cost would be far higher. Trust me, you would not want a different processing engine, different focus meters, and even control functions, the overlapping functions help improve overall quality.
Pick up a D7000/600 and then pick up a D800, the build quality difference is noticable immediately. I have dropped D300 right on pavement and it kept on ticking, do the same to a D7000, and expect a serious crack across the frame.
Little things like Bracket depth can make huge differences.
pixel depth, well, so far, I appreciate the d800 over the D700 for the details and ability to have increased crop flexibility. Sometimes the picture can be saved just by having the crop flexibility of moving the frame around the shot.
Glad he loves his D600, I would love to have a D600/D800 pairing, but already have a D7000, and the difference between a D7000 and D600 is not as much of a delta.
In the end, I am glad Nikon has such a wide set of choice,, just like in the film days, one had quality range from EM right to F3. I shot with FMs, FE, FAs, and F3.
It is true that photographers no longer have to spend a fortune for great shots. Heck, 5100 is on sell now for 500 dollars, and that is a holy cow. Think of it, a quality SLR for the price of a high price point and shoot. A d7000 sensor for half the price!
laughs,, my post is risking getting as long as Ken's,, but, I am not making any hidden endorsements.