If I were a Nikon afficionado, for the same price, why would I buy a Df over a D800?
Because you already own a D800 and D800E? I think this is marketed to wealthy hobbyists.
I know next to nothing about the Nikon system, but I think the appeal here may be that this camera is compatible with all the old Nikkor lenses. Something that Nikon had moved away from for all practical purposes on their more recent pro level cameras? Again, I'm not sure about this.
If this were the case it would make me sad in a way since this will never ever be a reasonable option for Canon. I have a bag full of nice old FD glass. I never liked the EOS/AF approach and only bought into it since with digital there was no other Canon choice available. And I didn't want to go the Nikon route a few years back because there are some Canon lenses for which Nikon doesn't have an equivalent. And a few other reasons at the time.
But had this thing been around and if it really takes old MF lenses without compromises (again, not sure about that) I would have had a very close look at this. I hope it's successful and hits a nerve with people who are sick of the spaceship cameras with touch screens and plastic lenses. I hope it's a trend because I would love to shoot again with a camera like that. Does it "take better pictures"? No. That was never the question. Remember the days when every camera loaded pretty much the same film? So that's not the point. It's how well a camera feels and handles and if you are happy to bring it along often.