I was telling about my experiences with overpowered dps in a heroic group yesterday. But last night I grouped with a nearly as well geared healer, and the poor guy was bored to death. Even my priest, with a far more mediocre gear score only needs to be half awake to heal through heroics. Any even more geared healer has to actually hope that the dps pull some aggro, because otherwise there is nothing even remotely interesting or challenging to do in a heroics group.
That got me thinking about how differently the three roles of the holy trinity of MMORPG group combat scale. DPS is pretty straightforward, the more powerful they get, the more damage they deal, thus the faster the mobs die. But there is an important relation with the power of the tank: For a group to support high dps, the tank needs to be able to hold aggro. Fortunately holding aggro is a function of the tank which isn't always gear-dependant. Many abilities of tank classes to "taunt" grab aggro in a way that does not depend on the damage output of the tank. Fortunately, because otherwise we could all just shoot our protection warriors, as they only deal half the damage of an equally geared paladin or death knight tank.
The other function of the tank, the one that scales a lot with gear, is to survive. Better gear mitigates damage better, and gives the tank a bigger buffer of health. So, the more powerful the tank, the less likely he is to die. Which is good, because when the tank dies, the wipe often isn't far behind. But this leads to a rather strange situation, because a tank isn't actually even watching his own health bar in a fight. The health of the tank is the responsability of the healer, not the tank. Thus a tank who is overgeared for an encounter makes life easier for the healer, not for himself. The tank's main activity is holding all that aggro, and that depends more on taunting the right targets, using the right increased threat abilities on the right targets. As some people remarked in a comment on a previous post, the tank is fighting more against the dps classes in his group than against the mobs. The more powerful the dps, the harder the job of the tank.
Thus even in a completely overgeared group doing heroics, both the tank and the dps are still keeping busy. The dps have a competition among themselves to top the damage meter, and the more damage they deal, the faster everybody gets his emblems. The tank is rather busy trying to compensate for all that aggro the huge damage produces, especially since the dps to top the damage meter are likely to jump into the fight too early, and not allow the tank to properly grab aggro before launching their aoe.
The odd man out is the healer. If the dps kill the mobs ultra-fast, and the tank is skilled enough to hold all the aggro and geared enough to mitigate much of the damage, then healer just has to throw the occasional heal over time on the tank. Otherwise there isn't much to do for him. He could try to add to the damage, but the damage he can do with a healing spec and gear ends up looking ridiculously low on the damage meter, even compared with the tank. Dealing damage for a healer in an overgeared group is more an occupation to keep him from falling asleep, not something that would actually be really useful. The most important addons for a healer in heroics are becoming Peggle and Bejeweled.