Many people believe that anybody who speaks English can become a successful ELS teacher. This is simply not true. You require a deep knowledge of linguistics, English grammar and experience with teaching English as a second language techniques, as well as a native-like command of English to be an exceptional ESL teacher. These are qualities most native English speakers simply don't have. It requires years of training and experience to reach the level of expertise you need to become an outstanding ESL teacher.
Every student is different, with her personal strengths and weaknesses that define the way she learns. A good teachers adapts to the needs of his students instead of sticking strictly to a rigid lesson plan. He must be able to measure the mood of the room and provide the stimulus his students need before they lose interest.
The fastest way to learn a language is to enjoy yourself while you do it. A great ESL teacher will prepare lessons full of fun activities, such as games, songs and role-play, to make learning English something enjoyable instead of a tedious succession of drills and boring exercises.
Not all students are linguistic geniuses. Some really struggle learning the English language and will make the same mistakes over and again. A great ESL teacher will accept this and be willing to explain things many times, using different techniques without showing signs of annoyance or making the student feel stupid.