That's a good question. I had/have a D300s prior to ordering a D600. I thought long and hard about it but ended up putting my D300s on consignment.
I know I'm losing the much faster frame rate and the AF sensor coverage (these being the main ones for me). But gaining the much better low light performance and perspective of FX (again my main reasons for moving to FX).
Just before I got my D600 I had a week long wildlife photography cruise on the BC Central Coast. The reach and frame rate of the D300s was as always a great advantage but when it came down to reviewing results critically the low light noise was just too much to stay with.
Now it's winter it'll be a few months until I get back to the wildlife and I know it will take a bit of an adjustment in shooting to get the most out of the D600 but the low noise and dynamic range I know are going to blow the D300s out of the water. So I'm okay with what admittedly was a difficult decision.
Having said that I will be keeping my D5100 as my lightweight body for hiking & climbing etc...