Haven't read the whole thread but here is my little bit.
Most instructors have problems fitting their approach to the student personality type.
The way one would teach a malleable 18 year old who (generally) accepts authority, at least from non-parents, is very different to the way you would teach a 50 year old business/professional person.
The former will accept whatever you tell him. But the latter is well used to dealing with all kinds of people professionally, and will not take any bu11sh*t, will spot a novice instructor from a mile away, and will give him a hard time (usually unintentionally).
I think many low-time instructors have problems with the latter type, and certainly this regularly comes out in the forums and in the instructor press (Flight Training News comes to mind) where, last time I read it, one well known instructor wrote an absolutely scathing piece denigrating business/prof types who turn up in a BMW with an attractive "blonde" woman companion
The unfortunate reality is that the BMW driver is more likely to have the funds and the time to hang in there to the end and eventually buy a plane and end up using his piece of paper for something useful.
Myself, I've had loads of instructors (due to changing schools, training in UK/USA, etc) and some were good and some were bad. I could spot the good ones now but no way could I have done so early on.
Interesting to note that the good ones were either retired ATPs or got airline jobs very fast. The bad ones are forever building hours.
My first instructor had 150 hours total time and even I wondered about that. He is still building hours 8 years later.