I recently moved from a D90 to the D600. I will agree with Stan about the D90 being very capable and that the pro level lenses significantly improve the images from the D90 vs images with the DX kit lenses.
One aspect of the D600 not discussed in this thread that I am growing to appreciate is the improved focusing. The additional focus points can be helpful. And, in my limited experience with the D600 for action I believe it is better for tracking moving subject (AF-C) for sports. Also, I like the playback option on the D600 that shows me the focus point used for the image. Getting this instantaneous feedback on focus point at the time of shooting sure beats finding out later when reviewing images in ViewNX2. If the D90 can display focus point on playback, someone please let me know that I have done poorly reading the manual.