This is in my opinion one of the main problems with PvP, in recent expansions healers have been VASTLY overbudget. Most well designed games know that healers should not be able to survive 1v1 with a dps indefinitely, and certainly without burning through their mana. They MUST require a group to support and peel for them to become valuable, otherwise they are simply too powerful and overbudget compared to the other specs.
They make the game unfun. They ruin the 2v2 bracket entirely because (as stated) they are vastly overbudget which makes dps/dps setups more or less completely unviable. They make battlegrounds unfun, which ever side has the most/better healers more or less wins by default. They make world pvp unfun, because the second a healer is involved the target becomes unkillable. They require band aid fixes like battle fatigue in arena to try and curb the fact that they are simply way too strong.
Why does a disc priest get a permanent 30% shieldwall simply for being in healer spec? A cloth spec with more baseline damage resistance than the heaviest plate, in addition to being able to switch heal schools so that interrupts don't even matter? Why are healers so tanky? The design is completely skewed. It's madness. (note that I think healers should be able to contribute more damage, to have at least a chance of killing a dps in exchange for being able to be killed themselves).
Voted Wotlk with MoP being a close second. Timeless Isle was best world pvp in the game so far, wotlk talent system was the best for diversity. Hybrid builds were the best fun.