The limiteds can even be made of ceramics... I'm pretty sure this was a limited run...
To answer the OP's question, I've always found it interesting that if you're talking about someone else's fancy plastic thing it is made of plastic, but your own fancy plastic thing is "polycarbonate".
In most modern, "not premium" lenses, most of the lens body is plastic.
There will be bits of metal here and there, and often people pitch a metal mounting surface as better.
The old, cheap kit DA-L 55-300 had a plastic mount, for example, while the "regular" DA55-300 and the new 55-300 PLM have metal mounts.
Metal filter threads are also considered a sign of higher quality, since they can hold up to more filter changes.
-Eric