Indeed it is not enough to be very good in your field. For example, being very good in English does not mean that you are a good teacher. A good teacher must also have other very relevent skills. One of them is COMMUNICATION. A good teacher must have very good communication (verbal, non-verbal) skills in order to convey what it is to be conveyed. S/he should take into account the level of the students. It's useless to explain something in an academic level for beginners and vice versa.