What makes a 'good' teacher?
A quote that I love, which I originally heard from Gerald J. Sussman, was that "the key to understanding complex systems is knowing what to ignore; what not to think about." Much in this spirit, I'll inevitably ignore many subtleties in the hope of elucidating a useful principle. I believe a 'good' teacher can be said to:
Not only understand the content of whatever it is they're teaching, but also to realise what cannot be assumed about a student's ability, and to act accordingly.
As you can see, this relates very neatly to the above quote. On the flip side of this, poorer teachers - lacking this understanding - will gloss over certain aspects of a topic. There could be multiple reasons for this; it may mean that there are gaps in the teacher's own knowledge, or, likely more often, that details are skipped because they are presumed trivial.
When dealing with multiple students, the background and ability of each will vary; the teacher's - admittedly challenging - role, is to evaluate these individual differences, and adapt their methods accordingly.
