I have been through a lot of riding instructors, both really good, and really bad.
The worst was a woman who did a "I teach everything" barn. I had done a riding camp with her one summer, and I like it, so I started taking regular lessons with her, and joined 4-H with her as the leader.
She was sooooo awful. She has a horrible personality. I know now that she purposely held people back so they would advance super slowly, therefore milking as much money from them as possible. She had obvious favorites, three girls who "taught" lessons for her, and they got the nicest horses, and they were the only ones allowed to actually ride. She wouldn't even let me canter because she felt I hadn't mastered the trot, even though I'd been riding for at least 5 years by this point. All the lessons consisted of was me trotting on the rail for an hour, while she sometimes told me something I should correct, and it was mainly "close your fingers". (still working on that one though, even after a broken finger
She was also a horrible horse trainer. Every horse a client brought in seemed to leave with more problems than when she started with it.
I'm surprised I stuck with her so long... but I enjoyed the 4-H program, even with her leering and bossing around everyone (GET ME A DRINK, I NEED A HOT DOG, etc.). I was there less than a year though! No idea how she's still in business though.
The best instructor I had was a woman who I was a working student for. I actually saw her riding her amazing stallion at the barn I was at at the time. I asked around about her, and eventually approached her about working for her, and she agreed. She didn't actually have a lessons program, she ran a warmblood breeding barn, had incredible horses, and was a nice woman. She explained everything very fully in calm tones, and we had clear goals every time we had a lesson.
I don't like the instructors that scream at you. I had one for many years, and although she was rarely screaming at me since I was one of her favorites, she was very condescending to students, and didn't explain things, but yet would get angry and insult the student when they didn't understand something.
If a barn is interested in getting and keeping clients, I think it is important for an instructor to have people skills. The teacher should try to gauge what kind of personality the student has, and base their method of teaching off that. I think it is very similar to how if a student in school has a creative and fun teacher, they learn better. I think this is especially true if the instructor plans to deal with a lot of beginner riders. I'm sure as a rider progresses to advance levels, they would put up with harsher teachers, because they are more interested in learning the skill than getting along. At that point it could be "get with the program, or get out"