The main issue with both Disc and Holy Pallies right now is having reliable, raid wide damage reduction that's unmatched by any other spec. And it's not like it comes at the expense of burst raid healing, both can do extremely well on those. But when the raid needs to survive huge burst of damage for several seconds - which is pretty much every single boss after the first three - they completely outclass anything else. No one cares if most specs can heal people up at roughly the same rate if they won't survive them in the first place.
What's especially funny is that Holy Priests are probably at their peak power (post tbc, since Disc didn't really "exist" back then), with high hps and two raid cooldowns, but still inferior to Disc. For yet another expansion. Screw whoever is in charge of balancing healing cooldowns. Maybe barrier and devotion should somehow start showing up on meters, so it's more obvious how big the differences are.