My personal reasons.
D4 - Professional - No problems with it - Not in my budget - End of story
D800 - I abuse my D300 for High FPS. The D700 at least hit 8FPS with grip. D800 not so much. That and cost are my stopping point. I might of splurged and went down the dark money pit. But its not "fast" enough.
D600 - Same issue as the D800 but worse with FPS. And now they remove the Multi-CAM 3500 engine. The 51 point is very nice. But the 39 point not so much. I abuse the autofocus engine. I normally use the 9-point on my D300. So you lose a little bit more on the top and bottom. So thats 2 points in my shooting style.
Also I don't have many Full Frame lenses. Had the D800 been the D700 and better in all aspects I might of jumped on it. The D700 is still very attractive even given its age I did order one in and had it in my hands before I returned it, it came in before I could really "afford" it and I did the responsible thing and returned it. The D600's been neutered enough.
I'm still waiting for my D300 replacement. I only hope they do make one and make it geared towards the market professional market that uses crop.