In my opinion, a good language lesson requires a teacher who is proficient in communicating using many different methods. The teacher should be able to convey one message in several ways while engaging the entire class. She should also be patient with her students and understanding of the challenges that normally arise while learning something new.
A good language lesson should include phrases that can be used immediately in daily life, colloquialisms, sentence structure, pronunciation, grammar and spelling. These may be taught by repetition, asking the class to repeat after the teacher, role-play, writing, word games and showing subtitled television programs or movies when the class is in need of a break from intense learning.
I think building confidence in students is very important to their progression in learning a new language. Announced tests and surprise quizzes help students and teacher discover how well the class is learning.
When a student excels, it should be celebrated. If a student is not progressing as fast as the rest of the class, extra tutoring should be offered. A good teacher will be aware of where her students are in the learning process while continuing to move the class forward as a whole.
In all, a good language lesson is challenging, engaging, productive and constantly builds on what has already been learned. A positive and patient approach by both student and teacher ensures a good learning experience.