What makes a pro body?

fft81 wrote:

In terms of ergonomics/durability/functionality, what makes a camera into a pro body?

My thoughts:

1. Ruggedness/durability. D3/D3s/D4 qualify, D800/D700 are close, but not quite there.

2. Speed. Frame rate as well as shutter lag. I'd call anything above 8fps as qualifying for pro body, but that would rule out D3X, which is a pro body...

3. Built in vertical grip? Not sure about this one, since it can be added to any body, but all pro D1/2/3/4 bodies have it.

4. Best low light performance. This is getting over rated IMHO, because the 2nd best is not that far worse and can be found in consumer bodies... We are not talking 5 stops difference of pro to consumer any more...

5. Metering/focus systems. Is there really a huge difference between d4 and d800?

So, why do people want a D4X when there is D800? I can understand a D4X at 54MPx, but would you still buy a D4X with 36MPx sensor like d800, but in D4 body? What frame rate does 36MPx d4X need to have to be a consideration vs cheaper d800?

Thoughts?