The answer seems to be experience. From Tenure, Turnover and the Quality of Teaching – New York Times.
The most important single influence is experience: first-year teachers are much less effective than others. The second year is significantly better, and by the fourth year, most teachers hit their stride.
But it’s not as clear cut as it sounds.
It is not entirely clear whether this experience effect is learning by doing (the more you teach, the more effective you become) or survival of the fittest (those who are not good at teaching tend to drop out early).
I think they miss the point though. What makes a good teacher? They assume it’s a teacher whose students improve on tests. (And that may be the case but it should not be assumed.) The business world tells us it’s all about metrics. People will tend to do what you measure them by. So if good teachers are those whose students get better scores on tests, teachers will work hard to make sure their students do well on tests.