What Makes a "Good" Tutoring Session?

Often, I walk away from a tutoring session (when I am the tutor and in sessions when I am the student), and feel a sense of accomplishment. I think, "That was great!" Other times, the session feels less fruitful. My question is, why? What makes us think a session is "good" or "bad"? In short, what makes a tutoring session work? I'm interested in hearing back from tutors, teachers, and, particularly, students!