As a wedding photographer, one thing that has always been lacking in my D200 is high ISO performance. The D300 did not give me enough improvement to justify the switch. I considered the D3, but the cost was prohibitive.
Now, the D700 seems to be priced low enough to get me to upgrade. The ability to shoot available light at ISO6400 and still have Noise Ninja recoverable images is the the thing that does it for me.
I only wish they would find a way to expand the focus areas to cover the entire viewfinder when using FX mode. I understand it was done to allow compatibility with DX lenses, and when in DX mode the focus areas do cover the entire frame.