I like Harris’ answer. These are general principles that govern any practice.
Professionalism is something that goes beyond actually following a code of conduct that one signs (though that is part of it). it is about honor, and integrity. One can be knowledgeable and experienced, but if one will cancel a class last minute to teach elsewhere for more money, or because one does not feel like going, one could hardly be in the truly great teacher category.
Knowledge of one’s discipline is important. In yoga this means study that goes beyond the bare minimum one would need to be a certified yoga teacher. It also, I think, means study that embraces the deep historical traditions of yoga, as well as of what modern science and practice offer.
Experience also makes a big difference. This does not mean that a new teacher will not be wonderful, but it is like what they say about doing body fat testing with the calipers“. the more practice one has doing actual testing, rather than just reading about how to test the more accurate the results become.
But we know that the best teachers have another quality. This is something that will be sharpened by study and experience, but is separate and not something that can be taught: passion, connection, charisma. I think it can go by many words, but we have all met teachers that reach us, that we remember, that make us understand in a deeper way.
In terms of some specifics to yoga look for someone who at least knows who Patanjali was. Look for someone who did not just take the cheapest ‘certification’ because they thought teaching yoga was a good way to make more money, or be able to teach more classes. Look for someone who teaches to those who are before them, who offers help and modifications. Look for someone who can go off script. Look for someone who does not seek to make you like them by making the class as hard as possible to stroke the egos, (their own and/or the students’). Look for someone who understands both the hard and the soft, who can lead you to places you did not think you could enter, by letting you lead yourself. Look for someone whose knowledge of breath or meditation is not gained primarily by reading wikipedia.
Although I don’t teach yoga, I believe what makes someone a good (or amazing) yoga instructor are the same principles that govern the rest of us. He or she needs to understand the concept of their teachings very well. The instructor needs to be a good communicator, be the example others would want to follow and be inspired by and have enough experience to be able to instruct in a safe way clients form all levels. Also keeping up with any continuing education and looking for ways to become better at what he/she does will make him/her better than others.
The instructor will also have to be dependable and reliable if he/she will keep the business growing.