They just hotfixed in a huge buff to priest mana. Rapture was increased from 2.5% of mana to 6% and holy concentration from 20% more spirit regen to 40%.
A discipline priest will get about 4 rapture ticks a minute. Over a five minute fight, at 3.5% additional maximum mana per tick, that means about 70% of their mana bar restored. In my current gear, that would be around 67k more mana to spend over the fight, or about 16.3% more mana to spend.
I'm not sure why this change was made at this point in the expansion. It could be that the developers were getting a lot of feedback from priests that suggested that they weren't powerful enough or that they were constantly running out of mana.
This is one of those times where my experience doesn't seem to line up with the experience other people are having. I've certainly run out of mana while healing as discipline, but if mana is supposed to be something then running out of mana has to be a possibility. I was able to heal heroics as soon as I hit the item level 329 requirement, even if my mana management strategy was occasionally to just hope for more crits. On our Argaloth kill I was the top healer.
I think what's really going on is that the majority of discipline priests, and the development team, are stuck in a Wrath of the Lich King style of healing. Though I still find myself doing it occasionally, I have almost entirely trained myself out of the reflex of throwing a Power Word: Shield on anyone who takes damage. I never use Flash Heal for any reason. I only cast Prayer of Mending when the damage distribution on the fight makes it sensible. I spend the majority of my time casting Greater Heal and Prayer of Healing. Basically, aside from Penance, there is extremely little overlap in how discipline priests should heal now and how they did heal before.
Most of the time buffs are accepted with open arms, but I'm a actually pretty skeptical of this one. If they want me to cast Power Word: Shield, then giving me more mana for my one cast every 15 seconds is not what they need to do. They need to either make Power Word: Shield large enough to warrant its cost and to provide a real life-saving benefit, or they need to make it cheap enough to justify throwing out regularly. This change reinforces the idea that you should cast Power Word: Shield on the tank every time Weakened Soul wears out and never cast it otherwise. Power Word: Shield without a Rapture proc is just as expensive and ineffective. Power Word: Shield with a rapture proc is now a significant net mana gain. It is a rotational ability that keeps your mana up, and more closely resembles Life Tap than Wrath of the Lich King Power Word: Shield. When a buff like this happens, it makes me worry about what upcoming nerf is going to compensate.