Not everything on the D600 is an improvement on D300 it seems. Check out strobist.blogspot.com.au/2012/09/nikon-d600-think-twice-before-you-jump.html
Slower flash sync speed compromises ability to use fill flash to balance ambient (apparently ... I do not own a D600). Not relevant for everyone, but a reminder to look for any compromises a new body might involve.