One thing I love about reading the great science fiction authors is comparing their special approaches to world-building. Ursula K. Le Guin supplies a complete philosophical, cultural, religious, even sexual profile for the societies she creates. William Gibson invents machines so intricate and realistic that you believe you could work them yourself. Robert A. Heinlein nails the social aspect, the intimate interaction of people in both familiar situations and those they're adapting to. Larry Nevin is particularly good at describing the creatures he invents and giving them well-rounded personalities that suit their physical selves.
The list goes on. Anyone care to add to it?