The healer is the most important member of a five man group in World of Warcraft, hands down. The healer is the backbone of the group. The healer is the motive force that allows things to proceed. If the tank is mediocre, the DPS good, and the healer good, you might progress or might not. If the tank is good, the DPS is good, and the healer is mediocre, you'll have a much harder time adjusting to any sudden surprises, If the tank and healer are both good, as long as the DPS can put out ANY DPS and there's no enrage timer, things will die.
This isn't meant to denigrate DPS or tanks. Heck, I'm a tank or DPS more than I'm a healer nowadays. But I believe strongly that healing is the silent motive force for a successful run, that a great deal of the pressure is placed on a healer's shoulders without the showy nature of the tanking role or the meter busting fun of the DPS roles. Healers are like the drummer, often ignored, but absolutely vital.
So here's your chance to agree or disagree, as well as to go even further. Are DPS the guitar? Is the tank the bass player? Have I gone completely insane? Do we even need that last question?