The price of the D600 is rumored to be very low - maybe as low as $1500.
The sensor inside the D600 will probably be 24MP (made by Sony, modified by Nikon).
The D600 will probably not have an internal AF motor, which means it will work only with AF-S lenses (just like the D3200 and D5100).
The D600 will have HD video.
Announcement before Photokina (September 2012).
One or more low-priced f/4 lenses will be announced with the D600. For example, Nikon recently filed a patent for a 24-70mm f/3.5-4.5 full frame lens which seems to be designed for a cheaper FX DSLR body.