Thing is, the D810 is actually a real competitor for the 5D III, as far as "general purpose DSLR" goes. It lacks in the ergo department, and is missing that 1 FPS, dunno about the AF system, although that was always pretty decent, but it is a LOT closer, and STILL has the better sensor IQ.
First of all, 'lacks in the ergo department' is very, very
subjective. Yes the grip on the D810 is still too small & not fat enough for my hands, and the D-pad is not as good as Canon's joystick. OTOH, there are many more customization options on the D810 than on my 5DIII. So there's a greater chance I can configure the D810 to suit my needs over the Canon. And let's not forget the complete lack of EC in M mode w/ Auto ISO when it comes to Canon. The 1Dx's implementation is so nonsensical it's almost lacking - you either have to use the LCD screen to use EC in M mode, or dedicate the Set button to activate EC in M mode. The latter removes one of the largest advantages of Canon ergonomics in my opinion - the ability to press 'Set' even in shooting mode to instantly check focus on your last shot. Why you can't adjust EC using the dedicated EC button in M mode baffles me to no end...
Also - if you consider this an element of 'ergonomics' - programmable Auto ISO itself enough reason to choose Nikon. When I'm switching primes during a wedding shoot, I don't want to have to remember to go in there and change my minimum shutter speed (and Canon's choice of 'minimum shutter speed' is often unsuitable). With Nikon, I simply choose slower to faster in 5 increments based on if I'm shooting static vs. moving subjects. Game-changing for the types of photography I do.
I agree, the D800 wasn't really a competitor for the 5D III...but the D810 is.
Now this I'd love some clarification on. Most people are pointing out how the D810 is not much of an improvement over the D800. DxO's own scores on image quality show this. So what suddenly makes the D810 a competitor to the 5DIII, but not the D800? The half-a-stop extra DR? The electronic 1st curtain [EFC] that can only be activated with mirror-up? Just curious exactly why you feel this way.
If I were to venture a guess - I'd say the EFC? I do wish, though, that Nikon had an option to implement EFC in all shooting modes with a short delay to allow mirror vibrations to dampen out. EFC only working in Mup mode is a bit silly - especially in Live View.
My bigger point here is that the D800 was just as big a contender. Not only b/c of its superior image quality, but also b/c of Programmable Auto ISO, Exposure Compensation in M mode, spot-metering linked to AF point, and 3D AF tracking. The latter allows one to simply use the center AF point to initiate focus on a desired subject, and allow the camera to track that subject across the frame, as well as along the Z/depth-axis. This (1) obviates the need to select the proper focus point, which is time consuming, and (2) tracks moving subjects like running brides across the frame. With every Canon save for the 1Dx, I have to manually select the AF point when I can't focus and recompose (24/1.4 and 35/1.4). Try doing that with a 4 month old baby that constantly moves around. For this particular scenario, I believe my focus hit rate went from something like 10% to 80% simply going from a 5DIII to the D810.
Canon's complete lack of a separate sensor for AF tracking in all but the 1Dx is rather egregious. And I, personally, find it difficult to use a 1Dx b/c of its weight/size that, with serious glass, puts it north of what I'm willing to tolerate. The 5D III uses some tricks to track subjects to make up for its lack of dedicated hardware - e.g. I believe it cross-references data from AF sensors to check if a subject at some depth moved from one focus point to another, and I think it also uses some info from its 63 zone metering system to help track subjects. But none of these approaches come near the (lateral, X-Y plane) tracking accuracy of a dedicated 91,000 pixel meter, or the entire imaging sensor itself in Sony SLT designs.
It's funny, if anything, I think the Nikon system is somewhat less
desirable now than it was a few years ago when the D800 was released. Why? B/c now Canon has some very, very fine lenses for it's system. The 16-35 f/4L IS & the 24-70 f/4L IS are great lenses for landscape photographers.