Pro level cameras (at least by Nikon standards)... look closely at the left knob... nothing but the quickess access to important functions... Completely do away with scene modes. So good luck if you can't figure out what's the best setting for sports, macro, landscape, etc etc... For this, I'd even categorise D300s as a Pro camera... (even NPS agrees with this). Full mag-alloy bodies allow the greatest chance of survival, imaging you're shooting a riot, a war, or Even Paris / Dakkar rally?... Fully sealed and weather proven...
Serious Enthusiast, D90 - D7000... Whilst D7000 has features that better D300s in many aspects.. it still provided the different shooting modes (assuming that the photographer may still need these assistance at times)... And you can't even lock the knob in case of accidental shifts... Half magnesium alloy isn't going to fully protect the camera when you're on important assignments... One half is going to protected, the other is going to be crushed if ran over by a car... D90 is completely plastic exterior, needless to say is no guarantee against tough shooting conditions (no insurance)...
Enthusiast / Beginner level... Less help screens to watch your settings/controls - that is assuming you don't need quick access to most of the settings, time isn't a great concern that you don't want too many buttons to confuse you. Scene Modes are a must...in case you don't know which is the best way to set for given situation...