John R.’s Answer
80-85% of the service problems are electrical in nature. So the ability to read schematics, understand them, and work them systematically is critical.
The other area is the science. Understanding heat transfer, the states of refrigerant, airflow etc..
A high proficiency in these areas enable a technician to identify and correct the root causes of a problem instead of just addressing the end result and being a "parts changer"