My english teacher in high school was an older, fairly harsh and not too charming lady. She was mostly disliked by my classmates. At one point they even organized a petition to replace her with someone else. I didn't sign it. In my perception, she wasn't unfair, just demanding and quite strict when it comes to grammar.

Later, whenever I met a new teacher (at university, in the workplace) lessons always seemed easy to me. I had to work hard for a good grade at high school, and her tests demanded precision and attention to detail. I started to appreciate her more.

Students are likely to prefer teachers with a pleasant attitude and lower difficulty level. A teacher easy to distract with an off-topic conversation. But at the end of the day, a teacher's job is to teach. Sure, a great teacher would combine that with charisma, ability to inspire and natural charm, but nobody's perfect.