A good language teacher must be competent. This requires a lot of study on the part of the teacher. If you are a foreign language teacher, you must have mastered the language you are teaching and the skills it takes to teach that language. As an English language teacher, you need to have a handle on the elements of English as well as having a background in education.
A good language teacher realizes there are several different learning methods. Some learn by hearing, some by seeing and some by speaking. Therefore, a good teacher will create a course that weaves together all of the various learning techniques. A good teacher is also willing to explore the different ways of teaching language, even the experimental ones, to provide her students with the best chance to learn.
People do not teach for money or fame. People teach because they have a passion for their chosen subject and for passing on their knowledge to students. A passion for teaching is an important part of the teaching process.
A good language teacher is not only skilled at creating assessments for her class, but she can also use those assessments to figure out how successful her teaching methods are. The teacher who understands some things work better than others and some things do not work at all will be able to make changes in her own teaching methods.
A good language teacher is going to be a benefit to her students because she will provide them with a path to knowledge. A teacher who is good at her job will guide a student toward the most comprehensive understanding of which he or she is capable.