In "Supervision in Social Work" (4th edition), Daniel Harkness and Alfred Kadushin describe the key qualities of a good supervisor as being friendly, approachable, competent and willing to put the time in to get the job done. Each of the various traits of a good supervisor fall into one of three categories: administrative, supportive and educative.
As an administrator, a good supervisor should be flexible enough to be an authority figure while still maintaining good interpersonal relationships with those he supervises. Communication, a willingness to adapt to change and the ability to implement and enforce procedures are also key.
As an educator, a good supervisor should be knowledgeable and willing to share that knowledge while facilitating the learning process with the supervisees. Other important traits include a positive attitude and the ability to instill a democratic atmosphere among the group.
As a supporter, a good supervisor can identify and publicly recognize good performance while offering those in the group a sense of empowerment. He is open to negative feedback and constructive criticism when appropriate, and he encourages open communication within the group.