This excerpt, in part was taken from an article in the Artist Daily and I agree with it completely.
What make a great instructor.
The first trait: the instructor's work has to not only be technically sound but it also has to move you.
You can't learn from someone who doesn't believe in their art but it's even more difficult if you don't believe in their art--at the very least, you need to be interested enough to want to learn more about it.
The way an instructor teaches how to think makes a big difference.
And most of all, a great instructor whatever--shares what they know.
This is the crux of finding great instruction.
You want someone who doesn't hold back and answers questions as well as offers assessments openly and thoughtfully.
Share knowledge is the key for me.... sometimes Tutors share, other times, it's like pulling teeth.