One of the worst things is a teacher who knows his material so well that he can't dumb it down enough to explain it to someone who literally has never seen that notation ever before.

A teacher without empathy is a terrible, awful thing that can turn students off so fast.

I went to get my degree later in life and I would butt heads with this one particular math teacher all the time who admitted was extremely intelligent, but the entire class was lost because he could just would not break from his predetermined notes and lesson plans. Everyone else in that class was 18, 19 or 20 years old and too naive or timid to voice their concerns. I was considerably older and paying dearly for these classes, so you better believe I refused to just let issues slide. I'm sure some teachers would think I was a nightmare student, but I wasn't trying to be disruptive - I was simply trying to learn and this guy was just bad at it with 3/4 of the class dropping out eventually.