From what was told by Nikon Pro Service when it was released;
The sensor is based on the sensor of the D4, and the difference/advantage of the Df is that the sensor and electronics are fully designed without video to provide a very clean image.
I'm sure you can find more if you go back and look when it was released.,
I ended up buying that Df camera yesterday. Thanks to everyone for your advice on that camera.
The Df I'm now an owner of is a black one, looks like new, with 2,000 shutter clicks. Got it for $1,000 USD.
I've been doing some test shooting to get a feel for the camera. Here's my initial feelings on it:
1. Just like you said Erik, it's a really "clean" image. Sharp with realistic color - not much to do to a Df file in post.
2. The grip is too small for me - but I ordered an aftermarket vertical battery grip for it which I expect will help.
3. I don't care for all the retro knobs on the top as I really don't use them for my style of photography much. I didn't buy this camera for the retro look, bought it for the incredible sensor that's in it.
4. The sensor is probably the best sensor that I've had in any of the many digital cameras I have owned.
5. It's been very cold here in northern Wisconsin USA lately. I did get out and shot some snow photos. I used a Zeiss Milvus ZF 85mm f/1.4 wide open for the below test shot. The top image is with no processing in post; the bottom image is after recovering the blow highlights in post with the Adobe Camera RAW Converter. The ability of the to recover blown out highlights from overexposed Df image files is very good!