In the latest post on the Tesla blog, PR director David Vespremi gives a very detailed yet easy to understand treatise on mass and vehicle dynamics. More specifically he discusses the distinctions between rotating, unsprung and sprung masses and how they impact ride and handling.
From the perspective of an EV where energy storage capacity is at a premium to begin with, all mass is bad, but some mass is worse than others, especially rotational and unsprung mass. In the course of his discussion, David also explains why Tesla chose to forgo wheel motors for the Roadster. If you have any friends that don't care about green cars but are considering spending money on over-sized wheels, you may want to forward David's article to help them understand why they should stick to eighteen inches or less.