Beyond always having solid control of enmity, beyond always picking up adds properly, beyond never causing people to take unnecessary damage, and proper cooldown usage according to the encounter, tank and healer performance can only be judged in pairs. Communicate with your healer and adjust your skill usage and gear itemization (if possible) to make the healer feel more comfortable. Similarly, the healers should do the same.

A healer can be the best healer for one tank yet be an absolute nightmare for another. Same for tanks.

This is actually also the fastest way to improve tanking and healing.