Well from my point of view healing is not much challenging at all nowadays (i can be wrong i know). It is simmilar to buffing as wrl/m: you can do minimum and it will be enough.
But what i think is most important is the fact, that only thing matters when it comes to heal is if you are able to keep your party alive. Maybe it is true that druid is easier to keep whole raid alive (i feel same) than priest, but to be honest everything depends of each player own playstyle.
I don't buy the supportive thing of classes like heal/tanks becouse only think that matters is not the fact that you can buff your pt, but if you are able to do your main job (heal/keep aggro).
In current state of endgame there is nothing such thing as challenge, even whet it comes to tank/heal/dps. People are doing "challenges" like tank with d/wrl, knight without subclass, s/k ;). Heal with full mage gear / solo heal anything. and also doing dps on every single combo that is available for each own items.
But going back to healers, maybe they need little skill rework/heal values rework/ anything rework? But right now it is rly hard to find good diection in balance becouse, noone likes "nerfing", and i don't think we can do it poperly without new challenge (new WB, new content etc.)
For what i know for sure is that some classes are better in healing than others, yet that is on almost every role in this game.
I hope we someday find the Golden mean to please every one
thx and see you in game