be sure to click on "view original size".
obviously the D600 has a lot more detail given the extra pixels. the D600 also has less moire (compare the tool cabinets right above the tree). on my (calibrated) monitors, the D600 has a much more pleasing (accurate?) white balance, the D90 has a bit of a green cast in comparison.
the D600 doesn't really blow the D90 out of the water in terms of dynamic range imho.
this doesn't surprise me, i've been very happy with the D90 for years *except* the white balance, don't always remember my expodisc and it can be a pain to get right in post.