Every major car show chases some kind of theme. A few years ago, all that manufacturers could talk about was LED headlights. Then it was hybrid technology. Then accident avoidance. At the Detroit Auto Show this year, we found twin-turbo V6's in everything from pickup trucks to supercars. Now, at the New York International Auto Show, the emergent attribute is lightness--as in, the judicious use of aluminum and other lightweight materials in vehicle construction to boost fuel efficiency and performance. Sluggish luxury sedans are ancient history. Welcome to the era of light-heavyweight automobiles.