I agree about the Mark III being better at taking my 8 EOS lenses over the D800. Don't know if it's been mentioned but small quibble where I also find the Mark III better:
Quiet Mode (vastly superior to D800 similar mode and very useful in application)
The Rear Grip area where the Thumb sits, the folks at Nikon seem to miss that. Your thumb just slides off. You have to squeeze the camera body to hold it securely. The Mark III address the ergonomic better for shooting.
I got to say I'm very impressed with the Mark III, it's a really nice camera to shoot and files are truly gorgeous! I come here and read these posts and agree that the D800 appears to be a better camera for less money. That makes it worse frankly. But I enjoy using the Mark III so much more than Mark II. It's truly superior on soo many key ways. I've read that there's not much difference in IQ between the two but to my eye, high ISO is better and love the camera NAILS focus consistently and is more responsive. My keeper rate is much, much higher with the Mark III.
I bit my lip and paid the $3500 and really enjoy the camera. But do feel it should be $2999.00. It's a great camera but I don't see what makes it worth $500 more than the D800, outside of possibly the focusing system.
The camera is paying for itself but as a consumer I like to feel like I'm getting good value. This purchase made me feel like I was paying a premium to stay 'in the club'. It wasn't a warm fuzzy feeling. Whereas, Nikon buyers feel like they are getting a bargain, 36megs, 200K shutter, same focusing system as D4, built-in flash, 100% viewfinder coverage, weather-sealing, great metering system for $500 less than the Mark III. Hard to argue with that comparison.
Canon has a HUGE video shooting base, and video shooters are used to paying far more for their cameras so they may have priced to camera based on that. I don't know, just throwing out an idea. But when I start to feel frustrated about what I paid for the camera, I pick it up and start to use and forget about that and just enjoy it. The camera is really a "tool" now, for me much more so than the Mark II was.