A top notch educator will be an expert in his/her field and will convey that knowledge and information while working with his/her students adapting to their individual ways learning. Some people are visual learners, others auditory, and other kinesthetic. The instructor/teacher/coach needs to have great teaching skills, be a great manager, a clear communicator, and a demonstrative example setter and should never settle for mediocrity.
However;
If you look at professional sports, how often do you see the most experienced athletes turn around at the end of their careers and start coaching? Almost never. In the NFL, there are a number of new head coaches who never played professional ball. Some of the coaches, like Josh McDaniels from the Denver Broncos are in their early thirties and have players on his team who are older and far more experienced than he is. The future will determine his success, but he's the man and is getting the big bucks for now.
Our children go to school everyday and are taught by young teachers with crispy new teaching degrees, and we assume that the institutes of higher education did an adequate job preparing them to teach our children. How many of these teachers actually have a background in English, mathematics, history, biology, or life in general?
It is the rare exception in scuba diving that someone, like Thal for example, has had a career in diving and science and then turns around and does serious instructing. Instead, I believe most of the Instructors out there are passionate about scuba diving and based on life experience, feel that they have what it takes to be an educator. They want to share their passion for diving with many other people, so they become Instructors.
They learn the material, and with some agencies how to teach the material. The minimum requirements needed to begin the IDC may be a bit low, but where do you draw the line? How many surgeries should a person have done BEFORE they are allowed in medical school? The key is internship.
Our dive center, for example requires a rigorous internship for our DMs, AIs, and new Instructors. If one of the new instructors has an extensive diving background, he/she will be given latitude, but that extensive background doesn't necessarily translate to the classroom atmosphere.
If you are a potential new student and have the luxury of interviewing your potential instructor, perhaps you can find an instructor who is very experienced, is actively instructing, is very thorough, safety conscious, demands excellence, and is actually a great teacher as well.