I don't link demanding and mean together as necessarily similar attributes. Sorry I missunderstood you. I think I prefer demanding and nice.
The general mode of teaching -- in the area of the arts generally -- has evolved over the past 25 years or so from a master/apprentice relationship (in the hierarchical sense) to a professional teacher/student relationship. Though it's dangerous to generalize, students used to be abused more in the past, and I don't mean so much physically as mentally: denigrated, mocked, used as gophers and servants generally (in painting, students were often expected to "finish" the work of the master) or just ignored. The general tendency now is to treat the student with more respect, and more clinically (i.e. more impersonally): neither as a friend nor as an inferior but as a client. In my area -- writing -- writers found their way into the teaching game relatively recently, and many of the very best writers were notoriously awful teachers. They often scared the hell out of their students -- an approach that might be good for a few, but was destructive for most. This approach has mostly vanished as writers have figured out the teaching game. I don't know to what extent any of this applies to the realm of private music lessons, but it might be a useful point of comparison.
I should add that some aspects of the master/apprentice relationship will always remain in the arts (unlike, say, mathematics, in which subject one can learn as much, up to a point, from a teaching assistant as from a Nobel laureate), but that relationship, at least in some of the arts, has been modified. Part of this process is due to the mechanisms of universities, which insist on such measures of accountability as teaching evaluations, etc. Some of the changes have occured because if you're surrounded by students in a modern university environment on a daily basis, life is just more pleasant if you treat them reasonably well.