The mentors I've had in the past were people who were: encouraging, generous with their knowledge and also unfinchingly honest in establishing expectations for the relationship -- e.g. I'm not your mom/dad, I'm not your enabler, but I will share knowledge and connections and set you straight when you start to wander off the path to your goals.
I think, too, a good mentor sees and nurtures a potential in the mentee that perhaps the latter doesn't fully recognize yet.
