Ultimately it should be keeping your group alive I would say.
That means your primary mission is keeping people cured from high mob targeted damage, specials, area effects, debilitating effects that can take people out of combat, etc.
We should also probably discuss what druid and shamans are supposed to be.
Right now I see each class's mission as the following:
Cleric: Primary healer for group and tank, able to get rid of debuffs like group members being held, blind, feebled, poisoned, or otherwise taken out of play.
Shaman: Sometimes soloer char that is a balance of healing and damage + offers strong group buffing effects (kind of like bard?)
Druid: Movement and pure damage
Mage: Utility and damage + strong area and pvp
------------
So what happened? Druids already had stoneskin, but then were buffed with cure massive and healing cloud.
Shamans got regenerate and healing wave.
So, in general in a group, you're going to cast stoneskin, healing cloud, cure massive, regenerate, or healing wave before you're going to see clerics dishing out cures or heals. So if there is a druid or a shaman in the group, they are the ones doing all group healing because their spells are safer and go out before cleric spells. Most tanks prefer to see a regen at good/fair keeping them in good HP, than falling to vbad and hoping the cleric doesn't miss a conc.
Just speaking logically, I would say the smart thing to do would be make the shaman and druid massive healing spells only able to be cast on themselves or their charmies. That puts healing back on clerics. Or maybe make it only able to be cast once a day on others or something like that. Or, if nerfs are at all scary, give those spells in a lower spell circle to clerics.
Basically clerics purpose should always have been keeping the group alive, but with other classes getting buffed far outside what they were supposed to be, clerics have lost their purpose.