Responsiveness is in there, too. If you have urgent questions and you wait a week for answers, your agent isn't giving you the service promised. If an agent becomes ill or has an emergency, there should be a backup contact for you with the agency -- and while sometimes they can't inform all their buyers right off, personally, if something really urgent comes up, someone in their office should contact you, IMO, asap.
I'm lucky -- my agent right now is wonderfully responsive and explains things to me in clear and straightforward ways. Then again, I bought from her almost two decades ago when we got the house, and I had such a good experience with her, even as the buyer, that having her as my seller's agent was a no-brainer.
Also keep in mind that a good agent will lay out realistic options for you on both sides, but isn't going to steamroll you. If your asking price is unrealistic, or your terms aren't the norm for the market, your agent should tell you so right off. Asking too much for your house is, from all I've seen, worse than not asking enough.
Mind you, I'm just another buyer/seller. IANAR. (I am not a realtor.)