I spent last summer shooting with a D5100. I mostly shot with the 18-200 VRII. Not so sure that the lens made it look inconspicuous. It is one heck of a lot lighter than the D300. Since I recently broke my collarbone, I have started using it again.
The D5100 is "SO" different from the D300. With the articulating monitor, many controls that Nikon users depend on are not available. Since I was also shooting video intermittently, I found it most convenient to shoot Aperture Priority. There is only one control wheel. Shooting in A, the wheel controls Aperture. Then I made sure the Fn button (in an awkward--for me--location on the left) was set to control ISO. I did get a lot of good shots. The higher MP count made up a little for the lack of longer glass.
As far as unobtrusive is concerned... One thing that I noticed was that the shutter sound irritated certain wildlife like Black Bears, and Bald Eagles. It helped me get a good shot of a Bald Eagle, causing it to look right at me before flying off. I got a lot of mean looks from bears. One actually took a few steps toward me. The locals moved away from me, with keys in their hands. The bears finally decided they were more interested in the Sockeye Salmon than me.