Teaching is not the same as parenting. Students go away and don't come back. You only get to know each other so much. And as invested as you may be in their growth, there are limitations. Still, there are similarities. Teaching, like parenting, forces you to rethink your old teachers, just as parenting forces you to rethink of your own parents. You try to imitate your good teachers and avoid the mistakes of your bad teachers.
There were always certain aspects of even my good teachers that I disliked. After six years of teaching at the college level, I've grown to be forgiving of some of what I saw in middle school, high school, and college as perceived failures, and far more unforgiving of other mistakes.
What I find unforgivable:
1. A refusal to change. No one ever teaches the same exact class twice. The best teachers are conscious of this fact. My best high school teacher, after 47 years, still reworks and hones every single one of his lessons. Teachers who focus on the same text over and over again, who refuse to ever rework or rethink any of their ideas are not teaching. They are not modeling the willingness to learn and grow they want from their students.
2. Consciously hurting students. Any authority figure will make mistakes and hurt those under their authority. This is a fact of human nature. The best authority figures avoid consciously hurting students. I never liked teachers who consciously bullied students, even those students I personally disliked.
3. Taking an inappropriate interest in students' lives outside the classroom. If a student wants to tell you about their relationships, friendships, or their issues with their parents, that's their choice and you should let them tell you those things if they want to tell them. But even in those situations, you need to maintain a certain professional distance. If they tell you about their emotional issues, such as their depression, you should try to lend a sympathetic ear. Let them say what they need to say -- there are good instruction sheets for just this situation -- but in the end, you need to highly suggest that they see the guidance counselor or the mental health services on campus. You don't get to be your students' BFFs. If you want to be their BFFs, wait at least one year after the final class. That's a very liberal time frame. (Obviously, you should take some interest in the students' lives outside the classroom if you suspect or know of abuse.)
4. Not taking into account the sources of troubling behavior. We now live in an age that is aware of mental disabilities. A student above me in high school earned the hatred of all of his teachers for his bizarre outbursts. They were telling stories about him in teachers' lounges for years and years. In retrospect, he almost certainly had Asperger's. I also know that he had a difficult home life, with mentally ill family members who in all likelihood verbally abused him. Oh, and he had ADHD. A student below me from elementary school was later diagnosed as autistic. At the time, teachers would scream in her face for "disrespecting" them. It's true we know more about autism today than we did then, but it should have been obvious that this was not an effective method of modifying her behavior.
What I find forgivable:
1. Occasional disorganization. You should go out of your way to be as organized as possible. But there will always be aspects of your classes that you thought you organized but did not. This happens to everyone, in the classroom and outside the classroom.
2. Delays in grading papers. Life interrupts. You almost always have more students than your pay grade.
3. Not giving the A minus to the student who is at the very high end of the B plus. The student with the B plus probably deserves a B, thanks to grade inflation. I also believe in the law of averages for every student. If you happen to have been unfair, another teacher has been more than generous. It balances out.
4. Being annoyed that all your students disagree with you. Sometimes you are the smartest person in the room. You still shouldn't be annoyed that they all disagree with you, but it's an understandable human reaction.
5. Being angry at that ONE student, the one who derails your class, the one who makes life miserable for their classmates, the loud, obnoxious one who always thinks s/he is the smartest person in the room. That doesn't mean you shouldn't take into account the sources of this behavior (see point 4 in the first list). But being angry is, again, an understandable human reaction. I knew a student who, from what I could tell, had no clear disabilities. He was just an arrogant, ugly, obnoxious jerk with the emotional intelligence of a 12-year-old mean girl. I'm sure he made some of my teachers long for the days of corporal punishment.