Everyone here mentions ISO and that's certainly a great benefit of the D700. The other is dynamic range. The dynamic range of the D700 and the D3/D3s is incredible. Folks aren't going to full-frame just for the ISO... You'll do just fine with the D300. It's a fantastic camera.
I had one for years and upgraded to the D300s and loved it too. When I finally started using a D700, the difference was obvious in the raw files. Now I just wish I'd have switched over earlier.
Now, I'm going through the same process due to the release of the D800e... Dooh.