As of right now, it seems as though the challenge is coming from healing mechanics, rather than boss mechanics. That's not how challenge should be formed. Challenge should be from difficult boss mechanics, not from flawed healing mechanics. I'm sure a lot of people have seen many healers complain about this new healing mechanic, with the exception of the few "elitists".
I've healed through TBC and Wrath. I enjoyed TBC healing and Wrath healing within the first week of release, but after that, it became too easy. This is not good either; a balance of challenge is needed. Also, the loved Dungeon Finder will cease to exist in the near future. If majority of healers decide not to queue anymore for random dungeons, whether it be for flawed healing mechanics, doubt in the PUG, etc.
Coming from a Holy Paladin's perspective, certain spells simply do not heal as much as they should. I have approx. 4600 SP, not great, but not terrible. My Holy Shock heals for approx. 8k, non-crits. I was healing that much pre-4.0. It seems as though Blizzard needs to buff scaling when it comes to spells, because right now it seems as though the SP to healing ratio is off.
Also, in heroics, even with decent CC, I seem to go OOM every other fight, and no, I do not just spam my big heal, Divine Light. I use my CDs, my smaller heals, my free heal, and my big heal when necessary. I still run OOM and it's still difficult to top people off, especially the tank when everyone else is taking AoE damage. Again, bringing me back to my point; difficulty should come from boss mechanics and not flawed healing mechanics.
TLDR: The difficulty in healing is coming from flawed healing, rather from difficult boss encounters. The SP to healing ratio seems to be way off.