A couple of fundamental traits:
1. Buyer agent: local expertise. An agent who can provide up to date market statistics, neighborhood and school information, local activities, and previous referrals.
2. Selling agent: Primarily their marketing plan such as where they will promote your home online (Trulia, Zillow, Personal website, individual website for the home, MLS, and active buyers.) In addition, recent sales, market projection, and previous referrals.
For both above, there is an intangible factor: personal interaction. The agent must be positive in attitude, approachable, available and willing to assist from beginning to end.