With 36 megapixels you can say you get into medium format territory. In is a low cost alternative to those that would like to have medium format but cannot afford it, or for those that can afford the 135 system is more versatile.
Concerning if 36 megapixels is "too much" "diffraction limited" etc, it is the exact same problem with medium format, and it have reached 80 megapixels so far.
You must have another mindset, with low res cameras you think it is kind of an error if the sensor is not setting the resolution limit. With higher resolution there are many other factors that come into play. You need to plan your DOF even for landscape pictures, you can often not expect that 100% of the picture is 100% sharp, but if you plan right the most important things are. Tilt-shift lenses should gain popularity from higher resolution, when you can plan your DOF just like large format view cameras. In shallow dof (people, fashion etc) photography the diffraction is a non-issue of course, but lens resolving power may become more of an issue.
An important feature the D800 lacks though as I see it is the mRAW and sRAW formats that Canon has. I want to shoot RAW 100% of the time, but if I shoot hand-held low light etc ~12 megapixels is suitable, I cannot hand-hold 36-megapixel-sharp (without flash). With high megapixel count a reduced RAW format makes the product much more all-around.