This seems to be the topic dejur so I will chime in a bit here.

The instructor should be a person with the correct temperment to teach another person without making them feel slighted. He also should be able to recognize what is happening during the flying session and be able to make corrections while teaching the finer points of that correction without making the student feel as though they just made a mistake that could ruin their whole RC flying career.

Most good instructors know enough to do that, as well as know enough about the finer points of the hobby to make sure other issues are covered, such as pre-flight, and post flight. A bit about batteries, charging, engine starting, tuning... not an absolute expert, but one that knows enough to make sure you do not make a major mistake and destroy your plane or hurt someone in the process.

As has been pointed out, I have seen and heard so many so called experts that I just would not like to fly with, let alone have instruct me. The one that instills confidence, teaches the correct way to fly, and allows a person to learn and grow is an important point and one well taken.

CGr