My current student teacher had a formal, half-day observation by her supervisor yesterday. At the conclusion her supervisor said she's doing great and this will probably be her last observation. I remember feeling quite frustrated when I was a student teacher and I felt as if I'd been "checked-off" by my university supervisors because I was already "good enough". I wanted them to do more to help me improve. Even if I was already good, I wanted to be great.
With the wisdom of a few years, I now believe that most of the substantial growth that we do as teachers comes about as a matter of self-reflection and that "greatness" comes over time. I really value colleagues who have shared commitments and similar philosophies, because they can foster conversations that encourage self-reflection. But at the end of the day, teaching is a very solitary profession. Most of our day-to-day work is done in isolation from other teachers and our growth depends on how often we look at our kids and our lessons and ask ourselves how things are going and what we might change.