first of all, congrats on such a thorough and thoughtful review of the 5D III. I found your perspective to be very enlightening given the range of what you shoot. Asking any camera to be ideal for everything from landscapes to weddings is quite a request and it appears the 5D III is up to the task.
However, your need to compare it to the Nikon D800 shows where the state of technology is today as well as our need to compare what we have with what else is out there. So, on that point, I'll have a few comments. Let me start off by saying that the D800 appears to be an amazing camera. But let me also point out that back in 2000 when I got into digital, I bought a Canon D30 (no, not a 30D) and Nikon didn't have a competitive product. When Canon came out with the 1D and then the 1Ds, Nikon didn't have a competitive product. So many of us have come up the digital curve with Canon because of our investment in glass that, for some, actually dates back to film days. And for that reason alone, it's important to see where Canon's "state-of-the-art" is by itself. And to understand that the market never stays static. If you think that Canon won't come up with a D800-beater in the near future, you haven't been paying attention. And then Nikon will come up with something "better". The beat will, indeed, go on.
The next issue is the one of does it do the job you need it to do. I base that evaluation on what I shoot, how I shoot and what products do I produce from those images. I've never owned a 1-Series Canon because both versions are more camera than I need based on all the criteria above. While I love my 7D -- because of the wildlife and action photography I do -- the 5D III is on my radar because of my show portraits and weddings. I like that combination. And I like the file sizes from both cameras. I don't need to invest in more hard drives simply to handle the 36 mpx files I shoot of my puppies playing in the back yard
So, from what I've read and from Preston's real-world experience of shooting show portraits, I'm thinking the 5D III is a pretty capable tool for those of us with an investment in Canon glass. If I didn't have that investment -- and wasn't happy with what those lenses produce -- I'd be hard-pressed to decide between Canon and Nikon these days. But, back in the infancy of pro or semi-pro DSLRs, there was only one choice. And it appears that the 5D III is an excellent evolution of that 3.4 mpx D30 I bought back in 2000.