I responded to this thread on the WoW Guild Relations Forums. In it, a paladin is complaining that he used to partner up with a shaman, and the two of them healed well together. However, a new priest was brought in, and now the healing in his raid has problems.
As I responded, it struck me how very different raid healing is from other areas of the game. For a tank, the game boils down to the mob and you. You need the healers, but it's not something you can really affect. Other tanks don't really interact with you, unless you need to hand off the mob in a tank rotation. You rely on the healers, but you don't really affect them. You just have to have faith that they will somehow keep you up.
DPS meanwhile is again alone, each person working on their optimal rotation. In the great scheme of things you work as a team, but when it comes down to doing your chunk of damage, you are alone.
Raid healing, on the other hand, is very much a team endeavour. You need to work with the other healers, and your healing style needs to mesh with theirs. As well, an individual healer's duties expand and contract with changes in the fight. If a healer goes down, the remaining healers need to pick up her duties. A good healer also needs to know how the other healers heal, so she can judge if it's appropriate to toss a heal or if it's better to wait for a Heal-over-Time to tick or a large heal to finish casting.
It's a curious mix of aggressiveness and trust. If you aren't healing aggressively, the other healers will take up the slack. But this will cause problems as they stretch too thin. Yet at the same time, if you heal too aggressively, you invalidate their heals, leading to large amounts of overhealing, and stretch yourself too thin.
Sometimes I think the hardest thing about raid healing is learning to NOT cast the heal, to trust that your fellow healers have the situation in hand.
You can feel it though, when you get into that rhythm. When your heals match up with your fellow healers. When you are healing steadily, but not getting stretched or falling behind. I find that the meters usually reflect this state. Lower overheal across the board, and a very even division of healing.