The ability to meter to old legacy MF lenses is in itself worth the cost difference. Then you have the user banks, the improved ISO performance, the 1/2 magnesium body, (some) weather sealing, the much bigger 100% viewfinder, and dual memory cards.... That is a lot of improvement for $700 difference.
To me the D7k is very very worth the price difference compared to the D90. Too bad my upgrade path is different... If I am looking to get a new DX cam, the D7k would be it.