Two excellent paragraphs:
Getting good teaching evaluations is not difficult. Teaching well is a complicated enterprise involving professorial expertise (students cannot measure this), articulation (students cannot measure this), judgment (students cannot measure this), the ability to measure what students apprehend (students cannot measure this), and fairness (only a minority of students can measure this).
Students can assess how much they enjoy classes in the near term; they cannot assess how much a class will mean to them in five, 10, or more years. How often I have heard university alumni declaim on how much a professor they had in years past meant to them without their realizing it at the time.