A good teacher.

[faa2010]

What qualities does a good teacher in piano need to have?

[m1469]

To try to put it succinctly (though there aren't enough words, in a way), very much the same sorts of qualities that make up a good musician, a good performer, a good person; and a good piece of music, a good performance, a good interaction .  Yes, on the surface those words seem to have much subjectivity, but the subjectivity becomes much less as the individuals' understanding of music becomes less subjective, as well.

[fleetfingers]

Ability to play the piano well
Love for the instrument and the music
Love for people
Patience
Ability to see the end result, and a clear method of how to get the student there
Flexibility and Pragmatism to adapt to each individual student and where they are in their cycle of progress and stagnation
Respect for the exchange of payment for the teacher's time and expertise; using the lesson time efficiently

The best quality a teacher has, in my view, is the ability to make a student's practice after a lesson more clear, precise, and productive than it would be had they missed that lesson.

[m1469]

Love for the instrument and the music
Love for people

I agree and think that these are the most fundamental.  However, oddly enough, learning can actually still take place (to some degree) even if these qualities are lacking.

The best quality a teacher has, in my view, is the ability to make a student's practice after a lesson more clear, precise, and productive than it would be had they missed that lesson.

This is a really good point, I'm glad I read your post .