A good teacher is a master of two subjects. The subject of study and pedagogics. A PhD in the subject of study and maybe one course of pedagogics will necessarily skew things a bit.
I would dare to say +95% of the teachers I had up until college were downright mediocre because they lacked one single paramount side of outstanding teachers: p-a-s-s-i-o-n. Sometimes this alone has the potential to drive one's pedagogical skills.
I think most teachers would say the same thing about the students they've taught. It's pretty hard to impart insight and understanding to someone who doesn't care and doesn't want to.
A passionate teacher is likely going to instill greater interest in a large number of students than that of one who does not convey the slightest amount of passion, dooming the poor disciples to such an ordeal -including the brightest ones.