I think two things make a great teacher: a love for the subject, and a love for the students.
But instead of trying to elaborate, I'll take an even more holistic approach. What is the point, anyway? What is a teacher supposed to do?
First of all, the teacher is preparing the student for life. Humans are a rare animal in that our babies come out extremely weak and helpless and stay that way for a long time (some people never become strong nor independent). It's a difficult balance in itself to draw the line between providing a safe learning environment and preparing them for real life. If the teacher loves the student, they will balance the line effectively.
Next, the teacher shares his or her knowledge with the student. That is probably the most superficial view of a teacher. A sort of database of information that youth need to know.
Also the teacher is an investor in society. By teaching these kids better and sooner, they will grow up with the potential to create better things than the generation before.
All in all, beyond the subject being taught, the teacher pushes the student beyond his or her normal capability. Out of love for the student. Out of love for society. No matter if it's film, electronics, or woods, a teacher has not fully done his job unless the student comes out of the class knowing he accomplished more than he thought he could.
I know, it seems like somewhat of a jumble of thoughts, but it was kind of a brainstorm to get me to that final point. It's not that I think design is that essential, but it's something I love, so it's the subject through which I will try to motivate kids to go beyond their previous capability. To not just survive, but to thrive, and actually give society more than they take.