Although I dont feel qualified to answer the point of what a good coach is in terms of their actual knowledge of the sport, I just wanted to make a point on a more simple level.
I think a lot of it comes down to whether or not you just click with your coach. All other things being equal in terms of knowledge it can simply be the difference in the way things are explained that can make things simply fall into place.
For example, albeit a bit off topic, I used to be a voluntary maths tutor for a charity that allowed people to just walk in off the street either on a one off basis or for regular sessions and help teach them about anything to do with numbers, from reading bus timetables to algebra and everything in between. When I joined I used to think that my way of explaining things would be understandable to everyone (yes I know, but I was only 21 at the time!
). However, if there was one thing I learned it was that you need to adapt your approach to the individual every time no matter how simple or complex the idea that you are trying to teach. Some people pick things up easily and are on your wavelength. For those on a different wavelength simply changing the way you teach, whether its introducing analogies or simply getting back to basics etc, can be all that is needed to help everything just slide into place.
I therefore think one of the differences between a good coach and a great coach is the ability to adapt your technique to your student and not get stuck in a "my way is the only way" mentality.
Thats just my opinion though
