I am going to go along with Brucie and Amos on this one. I was - maybe still am to some degree - guilty of all the faults which Jerry listed in his original posting. However, I like to think that any musician can improve. I also think that nothing is less likely to bring that about than a cruel attack on their confidence.
As an English teacher, I am very careful to make every student believe that they can speak English. That's half the battle. If they think they can, they do. Then I can move on to say, "I can help you to do this better."
I never had the privelege of meeting him, but I gather this was very much the basis of Rick Fielding's attitude to teaching. That's the sort of teacher I want to be.