It is hard to stick to your question without thinking about what makes a good learning experience. Yes, the teacher plays a significant role, but there's also a method they are taught to use and there's the student.
So, for the teacher it is important to have a class prepared before hand and be enthusiastic. That makes things easier, the message is better passed and people don't get bored. Another aspect is that teachers should ask students to do homeworks and review them and take exams on a regular basis.
Of course the student should study, there's no way to learn just by having a good teacher.
Where I would say there is a problem is with the method. It still requires improvement. One big problem with improving the method, at least in Europe, which sets a frame for many languages, is the Common European Framework of Reference for Languages of the EU. It is a big piece of knowledge, but it is therefore too rigid, too slow to be fixed or to evolve. Yes, the contents are structured based on the language needs, from basic things to more complex. However....
I would for instance give more importance to the following:
(i) Learning the sounds of a language: how to recognize them and how to pronounce. When I learnt Portuguese some sounds were very hard to pronounce or to understand. A simple word: avó or avô. In Spanish both would be pronounced the same. We, Spanish speakers, must be retrained to catch the difference between the two o's. So why start learning a bunch of words if we cannot understand what's being said or how to pronounce it. I was told at the time, that phonetics was an advanced level topic. Nonsense.
(ii) I would pay attention to frequencies and spaced repetition. On the first levels you are asked to learn body parts, fruits, vegetables. I don't remember using these words oft. So: I've already forgot many. On the contrary, other common words have not been covered.
A good question and difficult to answer. Every learner has different needs and one teacher could be great for one student and not suitable for another. A good teacher is definitely able to adjust his teaching method and material to the needs of his students. But not necessarily to all types of students. There are students who need 100% guidance, others who like to take the lead, there are the ones who want to have everything corrected, others who don't want to be corrected at all, there are students who just want to talk freely, others who want to have written assignments and grammar drills. It's difficult to find just the right teacher who can cater to your needs, so it helps if the student has a clear idea how (s)he wants to be taught. Honestly, every teacher will have a hard time with completely clueless students who know nothing more that they want to learn x language and expect the teacher to magically implant the knowledge in their heads without doing the work.
My overall experience with italki teachers has been great. Some fit my learning style more, some less but I didn't have any real bad experience. And my Spanish teacher is without a doubt the best language teacher I ever had. Since I'm a language teacher, it's not easy to impress me, but I learnt a lot during his lessons not only about Spanish but also how to use modern technology to enhance the learning and teaching experience.
I do prefer when my teachers have the experience of having learnt a foreign language themselves because then they can put themselves in the shoes of the students. Now, when I look for new teachers, I prefer teachers who know one of my target languages rather than English. I actually don't care about the English level of my teachers and it's actually negative for my language learning if the teacher speaks English too well because we talk too much in English. I want to be forced to speak the target language.
So, the real teacher, in my view, is the person who pays attention to his pupil because the pupil should have an encouragement to do sth, to go further in improving his language. What about me is that i was lucky enough to have practised with considerate teachers with excellent knowledge in Eng. They have a talent for making their pupils interested in the subject and this really counts for me. Of course, the professionalism takes the primary place but all people are so different and teachers have to be aware of how they can find a common language with their pupils.
In summary, the ideal language teacher for me is a person who has already gained experience in teaching and mastered his skills at a high level, this person can easily examine the knowledge of his pupil and make a statement about what curriculum is suitable for that person and only for them!