Subject knowledge is necessary, but it's not enough. For me it's about enthusiasm - for the subject, and for the students - and getting the students involved in the lesson, encouraging them to ask questions, while balancing that with control. That needs aptitude and skill. You can learn the latter but you need to have the former. Not everyone can be a good teacher.

And teachers should not feel they have to know everything - which makes them defensive and alienates students - but they should be honest and engage with the students. They should encourage questions and if a question is sometimes asked that they don't have the answer to - and this will happen to any teacher - they should be honest, praise the student for asking the question "that's a good question" and say they'll have an answer for the next lesson. Students that ask challenging questions usually have a good teacher.

Can I like Zombie's response twice?  I have seen teachers belittle students for not knowing an answer or asking a "silly" question. No student should ever be subjected to that. 

If we're talking about America, I think teachers ought to earn more money and there should be more work put into making it seem like a more respectful position to have. You might think that would encourage laziness and perhaps so, but I think if teachers had the same amount of societal respect that doctors or (sometimes) lawyers garner, you'd see more enthusiasm for the job and a lot more competition to try and become a better teacher. From what I saw when I went there, a lot of teachers fell into teaching positions as a last resort or because they took majors in college where it wasn't feasible to do anything else. That's potentially very toxic -- you have a combination of teachers who don't really want to teach and some students who obviously need to be motivated to learn. 

Enthusiasm, a zest for the job accounts for a lot. I can't count how many teachers told me outright that they "can't stand" children. That's absurd! The children really can't be blamed too harshly when they have to deal with people like that.

People say the teachers shouldn't have to do the parent's job of raising a child, and while I can respect that may be how Americans think even though I don't agree with it, the reality is that a major bulk of the day in the life of a child until they are an adult is spent in the company of teachers. Denying that they have an effect on how the child turns out seems stupid to me. On the other hand, monster parents who think their children can do no wrong and won't listen to a teacher who probably had a good idea in disciplining them aren't helping either.

Bottom line: if you care about something or someone, it's a good chance you'll be able to relate to them or it. I couldn't stand when a teacher wouldn't treat me like a valid human being just because I was a child and no should have to. A teacher who can relate to what kids are thinking and approach them on a level they feel comfortable with, while still retaining their authority, is a professional who is worth their weight in gold.