I started shooting digital with a D70...then upgraded to the D200 for the increased color saturation. Then I upgraded to the D300 for the improved higher ISO performance. All along I griped about the lack of a full frame option. Then the D700 appeared. Of course I upgraded again. So far, it's all I need. I rarely print larger than 16x20 and that's an easy task for the D700. The high ISO performance is good enough for shooting football games under the lights, and basketball games indoors. The color saturation and dynamic range are not lacking in any of my shots. That's about all the performance I need.
But the most important reason I don't plan to update is because I know how to use the camera. Every button and dial on the camera seems intuitive to me. When making in-camera adjustments, my fingers just seem to automagically move to all the right places. I can't really explain it any better. The D300 was similar, but I wasn't quite as comfortable with it. The D700 feels like it was made for me, personally.
Now that it's been discontinued, I have a choice to make: stick with my current lightweight kit (D5100/Sigma 10-20/Tamron 17-50/Sigma 50-150/Sigma 30 1.4), or sell the kit and pick up a second D700 body to use with my manual glass in a lightweight kit. I can probably wait a couple more months for the D600 to hit the market before I have to make a final choice. By then used prices will have fallen even more - which will make it an even more attractive option.