Re: G Series over D Series Nikon Lenses
To add to PolarBears' reply. As you might notice, the aperture setting is no longer controled on the lens but on the body. Nikon is making (I believe) a transition in which every new body coming out in the future, the aperture control is only going to be controled on the camera body and not on the lens. This makes controling the camera easier with one hand than two, aswell as being able to make finer adjustments (1/3 stops), instead of guessing on the aperture ring (1/2 stops).
With regards to the D series lens, yes, some have a metal ring, but, if you look at the AFS 24-85mm G lens, the mount is metal. The new AFS 70-200mm f2.8 G IF ED VR lens is also a G series, and the mount is metal.
Sooner or later, all the current lens that are D series will be replaced with G series lenses. The quality of glass and construction will be the same except for the missing aperture ring. Sorry, for the people with older style Nikon bodies in which you need the aperture ring to control the aperture.
Hope that helps.
PS: Don't it seem that Nikon is slowly following Canon's philosphy ie: lighter bodies, IS versus VR lenses, AFS versus USM.