A Frame Rate lower than 5.5FPS would have been a deal breaker for me.
I would use a D600 primarily for indoor sports. I upgraded from 4.5FPS to 6FPS when I went from the D90 to the D7000, and it made a huge difference for basketball. I could never go much below the 6FPS mark again. Gladly, the D600 shoots at 5.5FPS, which is just barely close enough. I was disappointed to find out that the battery grip doesn't improve frame rate like it does on the D300s. I guess they're saving that for the D600s.
I think everything else about the D600 is just about perfect for an entry level FX DSLR, except maybe the price. I would have liked to see it around $1,700-1,800. That is probably where it will be in two years, when I am ready to get one, so I'm even OK with the intro price.