Like most here I agree that dust is an issue, but one that every DSLR owner experiences. From my experience all of my cameras have had what I call an initial shutter oil break in period. Admittedly, the D600 did have quite a bit more dust, oil, gunk, whatever you want to call it a 200 shots than I thought it should. In fact I was a little disappointed it looked so bad, but not surprised since like you I had read ahead about this before buying it. I air cleaned it, but it did little. Used a brush, and it was a little better so I waited another 500 shots or so and did a wet clean because a couple of pretty big spots started to show up on my shots. I did some time lapse testing, ok I was playing and now at around 1600 shots there are some more but not nearly as much. Nothing compared to the beginning. I'll do another wet clean and see. I am hopeful that the worst is behind.
I am subscribing to the theory that Nikon's quality control was lacking when they made these and a little more lubricant was used than should have been.
So, the moral of the story is don't worry, the D600 is a great camera and I for one expect to get many many photos out of it (and hopefully a masterpiece or two) before I'm done!
---------------------------- NAS - Taking it one day at a time!