In the recent Blame the Healer thread, Markash linked to an interesting discussion that originally happened on the WoW forums, and was archived at MMO-Champion. And what I found especially interesting was the first part, quote: "Healer Competition: While most of PvE is cooperative, raid healing is stupidly competetive with poor support mechanics for multiple healers healing the same damage. It also creates an environment where faster spells are artificially more valued because it means your heal actually works. To overcome this with communication and mods takes an unproportional amount of effort."
In other words, if 3 dps classes hit the boss mob simultaneously for 10k damage each, it all adds up to 30k of damage. If 3 healers hit the main tank simultaneously with 10k heals, chances are that half of that is lost in "overhealing". The reason for that is basic math: A raid boss mob in Wrath of the Lich King has several million health. A main tank only has somewhere between 30k and 40k health. My priest currently heals for up to 9k with a Greater Heal, up to 14k on a crit. In other words, my crits heal a main tank for between a third and half of his total life. But with a casting time of over 2 seconds, I'm not waiting for the main tank to be below half before starting to cast. And if the main tank *was* down to half, another healer in the raid would probably start healing the main tank too, as even with healing assignments you never know what happened to the other healers. So overhealing happens all of the time, even in the best raid groups. Damage comes in irregular chunks, so healers need to keep everyone at or close full health, and that means that a good part of all heals is wasted. DPS is rarely wasted, because the best damage ability anyone has is still doing far less than 1% of the boss mobs health.
It would be wrong to judge healers on how much or little overhealing they dealt. Overhealing is the lesser evil, much better than letting the main tank die. As holy spec priest I'm often on raid healing duty nowadays, where I can use the "smart" spell circle of healing (until the nerf next patch), which automatically chooses wounded targets if in range. Way less overhealing, and much higher up on any healing meter than lets say a discipline priest keeping the main tank alive. But it would be silly to believe I would be doing a better job because of those better score. Healing meters are misleading. The only important thing is whether everyone is alive at the end of the fight. Avoiding overhealing ultimately falls under the category of mana conservation, which is important for long fights, so somebody who is overhealing far too much risks letting his target die later when he is out of mana. But trying to heal in a way which completely avoids any overhealing is ultimately the bigger risk. The 2 to 3 seconds a priest's greater heal needs as casting time is a long time in a raid, and from the moment you decide to start casting to the time the spell lands, a lot of things can happen. Using faster heals is less mana efficient, so avoiding overhealing that way won't help either.
I'm not sure how Blizzard could redesign healing to be less competitive, to avoid several healers working at cross-purposes to each other in a raid. I'd love a talent that gives me back part of my wasted mana when I overheal, but that would take away a lot of the skill currently needed for healing. Having some sort of display of which healer is casting what spell on what target sounds nice, but is probably too confusing in the heat of the battle. Maybe one interesting option would be to convert overhealing into temporary health. Lets say half of the points I overheal, up to a certain limit based on the total health of the target, remain on the target as damage shield, lasting 3 seconds. That way the points aren't totally wasted, because at least they enable all the healers to heal a bit less for the next 3 seconds on that target.
How would you redesign healing, or add new talents and abilities, to avoid overhealing?