Canon and Nikon are by far not the only brands that make great cameras

so don't forget to add a few more brands to your poll - Olympus, Pentax, Panasonic, just to name a few.
I went from Canon to Nikon, because I usually need a lot of dynamic range (it's just the way I shoot, and which scenes I like to take pictures of), and in that respect, the d600 and the d800 are currently the kings of the hill. Also, the d600 seems to have been less 'slimmed down' in terms of features than the 6d. I felt I got a better deal (more high-class features) for the money with the d600 than with the 6d. And the d800 just has more dynamic range than the 5d Mk3.
But it was no upheaval for me at all. I had also been considering several other cameras, and in the end bought the one which gave me the best overall package for my particular requirements.
If another brand should at one time become as much better in those aspects which for me are key aspects, then I will switch to that brand without any regrets.
I have friends who use Canons (5d Mk2 and Mk3), and are (rightfully) happy with these, and I have a friend who takes great pics with an OMD - he likes it because he likes street photography, and the OMD combines great image quality and very useful features with a small size.
The d800 and d600 are good in those areas which are important to me. That's all there is to it.