I don't think anyone can quibble about the build quality of any of the four bodies so I don't see any advantages there.
Comparing them all at http://camerasize.com/
is pretty telling really.
Nothing about the size makes me go wow, it is still a DSLR with most of the size and weight to it.
It does support a very very large range of lenses. that will be a big plus to those who own some of the older Nikkor glass. Seeing as not all AI or AIS glass was of spectacular quality ( just the same way as not all D and G lenses are spectacular ) I feel that appeal is limited.
The Pentax K3 also mounts an extremely long and varied lineage of lenses, some of which are truly great bits of gear. The Pentax also has in body anti vibration measures which I guess further enhances the appeal of older lenses when shake reduction is required.
Rather obviously we are comparing two 35mm format sensor cameras against two APSC format sensor cameras but megapickle for megapickle at those sizes does the 35mm format hold a massive advantage?
Yep, it has "retro" styling with the visual design. Yes it has "retro" dials to control the basics needed for an exposure. It also has "modern" buttons and dials to control things that the "retro" dials cannot. I observed before that it seems a rather cluttered way of doing things so I view the "retro" features as more aimed at the hipster brigade.
Yes, it will be a well made camera that will take wonderful images and have a big appeal to some sections of the photographic world but I don't see it as something that appeals to me.
Oh yeah, then there is the price ------ reckon you could buy a new K3 Pentax and at least one really good lens for the same price as the DF body.