I run a druid healer using ISBoxer and regions to heal with Vuhdo and it works really well. I cannot see how a druid would be more or less difficult to box than other healers BUT I have not boxed with either a shaman or a priest. Druids shine for their mobility while healing.
I have two warlocks in my main group but if I were going for sheer raw dps I would go arcane mage, not as many extra buffs but blows up stuff really fast