Here's a cutaway of a G series lens:
Looks like the outer casing is plastic but the mounts for the elements themselves look to be metal. Also the G lenses are weather sealed, the D are not, at least to my knowledge. Since focusing is internal on the G lenses, filters such as polarizers are not affected by lens rotation. One can hold the lens without part of it moving while autofocusing, and one can also grab the focus ring at any time to override the autofocus to focus manually without having to switch the lens and camera switches to manual. The optics are also upgraded from the D series at least insofar as being new designs. That all being said, I am by no means an expert on the D series so I'll defer to those who have greater experience with the different Nikon autofocus designs.
The G series would be a downgrade in terms of materials of the exterior of the lens, but I think this was a decision made because the internal focus mechanism requires a larger lens barrel and that without weight-saving measures such as addition of plastic on non-critical areas, the lenses would be tanks to handle. Would I prefer the greater robustness of the D series? Certainly.