The long and short of it is that teaching is both an art and a science. There are great artists in ski instruction within every national association -- and outside as well. But not even those who are most accomplished in the science of ski teaching will be great artists at it.To followup on my "empathy" comment above. What I mean is that a great teacher has to see and understand what their students know and are able to do. They then need to determine what the most important skill or skills to focus on are. Then they need to figure out how to transfer that knowledge to the student. To do this is usually trial and error as students "get it" through different perceptual and understanding channels.
So a great practitioner (world cup racer, world class musician, etc.) starts with a ton of knowledge, but that comes before the first step above - which is seeing where there student is at. That is not teaching skills. Teaching is a skill in and of itself. I know this from my professional life and experience which has all been in education and training.
Having a great eye for movement analysis is not the whole ball of wax, seeing what a skier is doing does not tell you what they need to do differently first -- and how to get that across. I have had things that I do pointed out to me where I was literally angry that no one had ever told me before. Examiner level and Level III trainers who had lots of great things to share, but no ability to look at me and say "OK, first off you need to change this."
I think I've had a couple of those from PSIA in 15 years. Stan Wilkes is one.
Only in the last 2 or 3 years when I worked with non PSIA trainers did I start to find what I needed to know.
@Steve, you hit the nail on the head that what often separates a good instructor from a great one is to know what your student needs when -- and how to communicate it to the student so that they understand the why, what, and how. The structure of the instruction environment can enable or inhibit the ability to give a student what they need at the current time. It's easier in a private lesson environment, more difficult in a group setting. And in instructor clinics it may be even more difficult.