Though I am not currently in the market for a camera, I was about 4 months ago when I finally chose the D7000.
Like many others here, I seriously considered whether FX was worth the higher cost not only for the body but for the entire system, including good full-frame lenses. As a serious hobbyist at best with a $2,000 total system budget, I could not justify FX (though I did look VERY closely at getting a gently used D700 at one point).
That said, deal breakers for me on the D600 would be similar to what they would have been on the D7000: No sub-dial, lack of dedicated buttons for common adjustments, single card slot, poor low light/high ISO performance, and lack of built-in auto-focus motor.
Indeed, the D5100 falls short in 4 of those 5 areas, which is why I decided to spend extra $ for the D7000. I say "extra money" referring to the price of the camera body, as I could have spent far more buying the D5100 and then buying AF-S lenses to replace the AF lenses I already owned and liked which can be used on the D7000.
It was a great lesson in total system cost as opposed to simply the cost of the body itself, which often ends up being one of the least expensive parts of one's entire setup.