I have always said people tend to think more about what bread to buy than what Realtor to choose. This article by Joanne Kaufman in a recent New York Times hits on many important points.
Sellers should choose an agent who can price a property correctly, market it effectively, and negotiate well. Buyers should choose a Realtor who knows the area well, and the intricacies of the area of interest. A Realtor who listens well is important. It also touches on the benefit of working with a single Realtor, not a team of Realtors when buying.