Does anybody think that in all seriousness NOT having a D800 is going to cost them work, money or enjoyment?
Great question! I chose the 5D3 for various reasons, but I think I can IMAGINE two people who might answer "yes."
1. The "pro" who either doesn't want to, or cannot, properly frame shots while capturing a decisive moment. With the D800 he'll be able to crop the shot in post to make it look like it was properly shot with a 5D3. If this person "only" had 22MP he would not sell his work because he'd have to crop it to 6MP or something.
2. Have you shot with or seen the files from a medium format camera? I got to check out the Phase One earlier in the year and just looking at those files makes a person smile. The single D800 file I've played with sort of had that feeling too it. So a pixel peeper, camera trekkie kind of guy may actually enjoy their D800 much more than other (in class) cameras simply because of the insane amount of detail that is captured. I'm not convinced that translates to any meaningful benefit in print however.
The thread asks if the 5D3 is better at anything over the D800. My answer? When I had the chance to play with both of them I didn't even touch the D800. The 5D3 sells itself because of expectation and reputation. The only thing that the D800 seems to best the 5D3 at is a failed (IMO) marketing move of having 36MP which are at least 18 more than we really "need" and I put that in quotes because I'm not convinced we need that many even.
The 5D3 is better at fitting into my workflow, both by giving me the tools I need and not getting in the way with "features" I don't.
If you're a Nikon guy with lots of glass get the D800, if you're Canon get the 5D3 just like every credible review has said neither camera is strong enough to make a person who is at all invested in the product line switch. They are both truly great cameras. The 5D3 is just better.
It's better because it fits me.