Interesting question.
Photographic Lenses to the best of my knowledge are always and will always be glass. Plastic just does not the clarity for photographic lenses and the plastic is too easily scratched.
The body of the lens could be a lighter alloy or possibly plastic on some lenses.
The density of the glass could also be lighter.

There are some issues between some G series, D series and N and F series lenses. Follow this like for some more information:
http://www.nikonlinks.com/unklbil/b...

You might have to contact Nikon directly. Nikon lenses are not as easily compatible as Canon and Sony/Minolta.

In several ways the 12mp compact could still be less quality than a 6mp Digital-SLR.
Due to the physical size of the sensor and the lens the compact has a tendency to have chromatic aberrations and purple fringes.
These aberrations look like red, blue, purple and sometimes green edges outlining some objects in a photo.
These are more noticeable when the photo is enlarged or zoomed in on using a photo editing program.
Also images above 200iso on a compact camera compared to a Digital-SLR are grainier(noisier).

Not sure what product you are asking about.
There is a Nikon D5000 that is a Digital-SLR. There is a Nikon Coolpix 5000 that is a 5 megapixel compact camera. There is a Nikon 5000DX module for promaster flash units.