While I'm not exactly surprised by Nikon's announcement, I'm pleased to see that they've put together a 1970s-style camera body.
When I was selling the Nikon F2, we had a Nikkormat body, which was their less expensive, smaller brand. It was easily usable without the crane and tripod. Compared to the Olympus OM-1N, it was still quite heavy.
When I first got a dSLR in 2004, I wondered why they couldn't do something in the same size as the 1970s bodies. It seemed ridiculous to have a smaller sensor (APS-C or Four-Thirds) in a body larger than 135 format. Then, I mounted a longer, bigger lens and realized how much more comfortable these dSLRs were. The grip kept them from being slippery, too. Using a 4x telephoto zoom, and relaxing and holding everything with only fingertips on the grip says a lot of progress has been made.
So, Nikon brought a camera to market that looks very much like the 1970s, adding a couple of LCDs and a lot of automatic features. The price is up there, but not outrageous for what you get. At roughly US$2750, you get their top of the line 135 format/FX sensor. (If you think higher pixel density leads to better image quality, you need to think again.) Combine that with the D610's auto focus unit, a single card slot, and all the dials you can handle, and you've got a great casual shooter than wipes Sony's A7/A7R duo right off the camera shop floor, except for size, of course.
No, it's not likely that the same people are going to be driven to Sony and Nikon bodies, or anything with the retro look. I suspect a lot of people in their 70s will buy the Nikon Df, if for nothing than nostalgia, as they probably have an old Nikon body in the closet.
Why do I damn the Df with the word "casual"? You'll see that the first time you mount the 70-200mm f/2.8 lens to it without a tripod handy. It's the same reason I call most of the micro Four-Thirds bodies casual--you'll need to take more care if you want to work handheld with an unbalanced pairing of body and lens. Thankfully, Fuji has been thoughtful with their X-series bodies and only provided an adapter for Leica M-mount lenses, which are small lenses. In the 1970s, it wasn't a huge problem because there weren't so many huge lenses, unless you were using Medium Format.
If you fit into that category where you take the time to take the shot, I suspect that you'll be richly rewarded by some very amazing image quality with the best lenses. If you dig the bottom of the barrel and come out with a 40 year old Sigma lens, you'll wonder why there was any fuss about the Nikon Df. Using high quality lenses with it will make you admire the D4 even more.
Thinking back a bit to the D610 announcement, I see where it makes more sense now that the update came about the same time as the Df, especially with the update to the Expeed 3 processor and faster burst rate.
Update 2013.11.11: Saturday, I was at a local camera store for a workshop, and I was discussing something later and I noticed a couple of the film bodies from the 1970s and 1980s, and they brought back a sense of nostalgia, having sold such equipment in my past. Unfortunately, most who spent US$500 aren't as likely to spend US$2750 for nostalgia, I expect.
Update 2013.11.24: Reading What Digital Camera's impressions made me smile when they said that the camera body was larger than expected, even though it's the smallest Nikon FX body. Nikon built huge camera bodies way back when. It was only after Olympus started to take business away that they responded with smaller bodies. The article was suggesting that the balance wasn't very good with zoom lenses. They should go back to the 1970s when none of the camera bodies had much of a grip at all. If you wanted stability, you bought a tripod.
Update 2014.12.18: I wonder if Nikon has sold more than a few thousand of these bodies. I don't see much about this body any longer. I suppose Nikon enthusiasts and professional photographers from the 1970s bought quite a few of them and that buying spree was over quickly. What value can you place on nostalgia?