After shooting with the Nikon D700 for just a few days I can already notice a big difference in performance between the former and the D300. Shooting on a partly cloudy day with the D300 was almost painful at times, the noise and loss of detail at high ISO could take a lot of "pop" out of your images. Now shooting in those same conditions with the D700 is a breath of fresh air, thanks to being able to shoot at ISO800-1600 with no fear of noise or loss of any major detail. Of course the draw back of shooting with a FX body is that you loose the "reach" of the DX body, but I think the improvement in high ISO performance is well worth that. I wouldn't even try to heavily crop ISO1600+ images from the D300 because noise made the images look flat and usually a little boring.
Images taken with the D700 have a different feel than the files from the crop sensor bodies that I have used in the past. First of all, the files seem to capture colour and contrast much better, even in unedited RAW files. Secondly, images just seem to have more pop to them, maybe due to the narrower depth of field that the 35mm frame offers? There is more separation between the area of focus and the background, and this becomes especially noticeable with telephoto and fast aperture lenses. I'll have more to say about shooting with the D700 in the days to come.