The BIG diff here is Sony brought 35mm sensor into tiny body at reasonable prices.
True, but the OM-D E-M5 is part of an existing ecosystem of small, high quality lenses designed to match up with that small body. A FF sensor in a tiny body with a big lens means the tiny body is at best no real benefit, and more often a hinderance when handling the combination, basically the same as sticking an EOS M behind a 24-70/2.8 or 70-200 lens.
The appeal of the a7R to Canon shooters is that it works with their existing lenses, and provides a higher resolution sensor with better low ISO DR than any current Canon body. The appeal of the a7R to Nikon shooters is…not much, really. The appeal of the a7R to Sony shooters may be substantial…I bet many of the 350 of them are profoundly interested! 
LOL...It's hard for people to step out their comfort zone. Regardless what might happen to Sony FF mirrorless cameras in the future, I admired them for bringing wonderful consumer high-end cameras to the market (RX100 II, RX1, and now A7).
I agree ... but in the case of Sony, there was no choice but to innovate, as they are a loss making company (due to poor product support) ... I would be happy to buy the Sony FF mirrorless camera
if I know for certain that they will continue with this product without discarding it (and its customers) down the line ... first they said in Camera Stabilization is what they stand for, then they ditch it for this FF mirrorless without in-camera-stabilization, now there is no way to know for sure (given Sony's past) that they will not put in-camera-stabilization in a future model (screwing the early adopters) or continue on to make more image stabilized lenses. I want Sony to succeed and I thy they can, if they don't ditch their customers and provide sustained customer support. At the moment I am not really convinced by the FE lenses (except for the tiniest one), all those lenses look big for a camera of this size, especially the white one. If Sony succeeds in this, it will be good for the industry and we get to benefit from increased competition.