Healing is the easier but often the more thankless jobs, compared to tanking and dpsing.
Imz likes to graphically explain his situations, looking here and there, but what Imz is trying to say, and I agree, is that both tanks and healer must adapt to each other play style.
A good healer, in a 5 men, should learn to adapt to how best heal the tank. Pally tanks take the quickest burst of damage at boss, so BIG heals, where Druid and Warrior tanks take less, hence HOTs are generally more effective. Also play style. Crazy pally tanks like Imzra loves to aggro whole instance, and tank 173453548 mobs at one go, so have to be careful when to heal, healing too early, u pull aggro.
Like-wise, a tank needs to also fully understand the healers. Pally healers can take good damage/bubble themselves, priests can fade/shield, Resto druids are the most iffy when it comes to 5 men, as any aggro means u have to pick them off immediately.