For me, a good language teacher keeps the learning realistic and fills the lesson with real world language. I really hate this kind of exercise:

Fill in the blanks using the present perfect:

1. He ____ the cat (see)

Instead, a real-life function of the present perfect is far more useful, like "How have you changed in the last year?". Language should not be manufactured in the lesson, it should naturally emerge from real-world ideas (not necessarily just from conversation either! It can be from videos, texts, etc.).

A second thing I expect is that the teacher be familiar with the massive amount of online resources out there. I used to learn Persian, not a common language for learning, and while online resources out were very hard to find they were hugely helpful.

Finally, I expect a teaching approach a little more sophisticated than "SPEAK!! Just SPEAK!"...if a teacher implies in their introduction video that just by chit-chatting everything else will come naturally then that really puts me off...

If my level is below intermediate, I need explanations in a language I can understand.

I don't like wasting study time to cover on my own the material I should have covered during class. 

Some times I have to discard good teachers just because their english is broken and I have to look up vocabulary and grammar on my own. A waste of valuable time.

Using only the target language doesn't work for me. I tried it for one language (daily, intensive, all immersive) and I'm barely A2.

I've noticed that there is no chance I can speak in real situations if I can't find TV or radio ressources in my target languages. No matter how much speaking practice I'm having in class.

I speak English and I'm learning Spanish. I prefer that my teacher use as little as possible of my native language.

When I really get stuck on something and can't understand it, I like to try to understand it by having a conversation with my teacher in the the target language. But I don't think this should be 100% absolute. There's no reason to insist on 100.0000%. Sometimes something slightly too complicated for me to understand in the target language, and she can explain it easily and quickly in my native language. It's better for me to understand an explanation in the target language than in my native language, but it's better for me to understand something in my native language than not to understand.

Frequent switching can be a problem. It takes a moment for my brain to "switch gears." When the language switches from the target language to my native language, I feel as if I've been drinking sparking water and suddenly the water has turned flat and tasteless.

I feel that it is a positive benefit for my teacher to know my native language, because it helps her point out similarities and differences, and helps her understand why I make certain mistakes.

I prefer a foreign teacher who teaches me speaking and listening skills. But reading skill and grammar, a teacher who can use my language is better.