A Therapist is a good teacher.
There are many well-intentioned therapists in the field but many of them just touch the surface. They listen to you and support you but they don't teach new life altering concepts. For instance what is love? How do we find more love in our lives? What is the purpose of life? A friend of mine whom I admire decided to go around and interview therapists and ask them this question, "What is neurosis?" She was very surprised to find that many therapists didn't have very thoughtful answers. A good therapist can answer a question like "what is neurosis" competently.
A good therapist genuinely will always treat you with the utmost respect. He or she understands that everything about you is totally respect-worthy, meaning that there are good reasons for everything you do. With this understanding they will be able to have compassion and empathy for your situation and realize that the job for the patient and therapist is to find the underlying causes of the problem.
A good therapist realizes that the therapy process is a collaborative process. He or she is not a manager or a recipe giver. The good therapeutic process is one where there is a caring collaboration between therapist and patient. They begin a thoughtful research process to understand the underlying causes of the problems.
A good therapist is honest and candid about themselves. They will not be hidden. They will not take a superior attitude. They realize that we are all in the same boat in life and that we all struggle with the basic issues on a continuum.
A good therapist practices @Changing Therapy. They realize that support therapy where the therapist just listens and supports the patient is only half the cure. @Changing Therapy, a superior process, introduces new concepts and offers the support to help the patient make life altering changes. Many therapists are afraid to talk about issues but it is an essential ingredient in quality therapy to create lasting change.
A good therapist has a good self concept and has spent many years working in analysis on their own issues so that they have a good solid understanding of themselves and their own problems.