My premise was that the 6D is capable of very good IQ - as the 5D2 already was 4 years ago. There's really no surprise about that, being a well-established technology.
What concerns me is how users' opinions vary awfully depending on the brand. In the numerous 5D3 vs D600/D800 debates, Nikon cameras where found faulty exactly in those segments in which the 6D is even faultier (e.g. AF, weather sealing, build quality, etc). All those valid arguments - apparently - no not apply to products with Canon branding. On top on that, the 6D cannot compensate with cutting edge sensor performance and it's a full 300€ more expensive than the D600.
All my favourite lenses are 3rd party (except the 135/2), so I very much don't care about the brand of my body. I'm still undecisive whether to buy a 5D3 or a D600, but if it was D600 or 6D, I would really have very few doubts. Considering the 6D is mainly aimed at crop-upgraders, I'm really dubious about what the concrete reasons for not switching system are.