When I pre-ordered, my 5d2, I told the salesman I hoped it would come in at 15-16 MP, which struck me as offering the best pixel density for 2008 technology. Given the relative strengths of the D700 and the 5d2, that seems like a reasonably astute shot in the dark for someone who isn't particularly tech-savvy. So. That's my credential for saying that something in the 20 MP range (18-22) strikes me as the sweet spot for current technology. Landscape photography is my first--but not my only--photographic passion, and I don't envy Nikon shooters their 36 megapixels. If the D700 demonstrated anything, it was that 12 very good MPs made a more adaptable camera than 21 good MPs, and the lower resolution was not a significant issue in many real-world shooting situations. (Even today, art directors who demand 30+ MP are indulging the fantasy that they work for a high end art publisher. There are no newspapers and almost no magazines that print to standards near that.)
MY personal photographic road map in retirement is to shoot enough local school events to provide the money and equipment to travel to the world's most exotic locations and become fabulously rich and famous for my calendar art. For that I'll need a camera that can handle the ISOs demanded in the dim, grim flourescent light of the middle school gyms and cafeterias where you find eighth grade basketball, fifth grade talent shows and Daddy-daughter dances. Outdoors, it won't have to have the frame rate of the cameras on the side lines at the Super Bowl or the Olympics, but it will need to be reasonably fast, with a high rate of AI servo hits for fast-moving sports like lacrosse, football and soccer. When I take it to the Tetons during the summer, it will have to have all the accurate color rendition, resolution and dynamic range you can squeeze out of ~20 MP. Oh yeah, and a 100% OVF with which to see it all.
In all seriousness, the speculative specs for the 5d3 suit my generalist purposes to a T.
That said, I haven't made up my mind yet about upgrading. I share one sentiment with several other contributors to this thread: what's currently competing for the dollars in my photographic budget is Canon's own upgraded lenses. I'll also have reservations if Canon prices the 5d3 $500 above the D800, and does so not because of demonstrably superior technology but in the belief that videographers locked in by the 5d2 will provide a profitable sales volume in spite of the price differential.
My decision about upgrading--like that of Neuroanatomist--will depend upon an evaluation of the full spec list--and probably a handful of reviews. (My options, however, will not include the 1DX, and I don't plan to pre-order as I did with the 5d2.)
There is one feature, however, that could make me take the plunge blindly. The search engine for this site shows surprisingly little on the subject, and I hope to open a thread on the matter in the next few days. Suppose Canon changed absolutely nothing else about the 5d2--including the much-maligned autofocus. If the 5d3 featured an honest (RAW) two-stop increase in dynamic range, I'd rob a bank yesterday to come up with the MSRP.