The old line (well, an old line [well, a line I used] ) in Wrath was that discipline priests don't heal, they shield. After all, holy paladins were better at tank healing. Restoration druids and holy priests were better at raid healing. Restoration shamans could swing between both and pop bloodlust. The only reason to have a discipline priest, if you had your pick of healer compositions, was for the mitigation they brought via absorbs. Mitigation is powerful...you can't heal someone back up after a lethal hit, but you can mitigate a hit down to non-lethality.
Time for the line to change? Discipline priests don't shield, they DPS. I mean, yes, the revolution got underway in Cataclysm, what with Evangelism, Archangel and Atonement. This suite of spells allowed discipline priests to strategically DPS via Smite and Holy Fire to do a little bit of healing and build up stacks that could be traded in for a healing buff at need.
However, the other signature healer-DPS ability was given to restoration shamans: Telluric Currents. This was a much simpler mechanic...no messy stacks to ensure don't fall off, no cooldown to time, no hoping that someone useful was in range of the smart heal. You cast a lightning bolt, you get some more mana (if you hit with the bolt). No muss, no fuss. Mana is fungible after all...you can trade it in for anything your heart desires. Another Greater Healing Wave, another Chain Heal, another Healing Rain...hey, big spender, dig this blender.
But, hey, funny thing happened on the way to the next expansion: discipline priests got Telluric Currents too. Well, it has a different, more World of Warcraft priestly name: Power Word: Telluric Currents. No, wait, that's not right. Power Word: Solace. It'll probably also be balanced separately.
However, conceptually, it's pretty much the same. Cast a DPS spell, get back some mana. With mana expected to be as heavily restricted for healers as soda in New York (too much mana consumption, after all, has adverse effects to your health) in the early going of Mists of Pandaria, this could be a pretty important ability for priests (I should note that any sort of priest could take this ability, but since it better fits my narrative, I'm going to stick with discipline priests). It is actually in the same tier of talents as other mana sustainability talents (it replaced Archangel, which will now be baseline for discipline priests) like Surge of Light and Mindbender, but this is the ability in which higher skillcap can result in higher reward.
Mindbender is just an improved Shadowfiend. It's a fire-and-forget several times an encounter. Surge of Light, as always, is a proc that is largely out of your control. Power Word: Telluric Currents is an active ability who's only limit is your imagination (and the hard coded limits). The better able you are at weaving non-healing spells into your healing rhythm, the more mana you will have to play with in fueling your aforementioned healing rhythm.
So, from a discipline priest perspective, you're looking at using Holy Fire, Smite and Penance (Smite will likely drop out in any serious healing situation) on monsters to build Evangelism stacks and do some Atonement healing along the way, and Power Word: Telluric Currents on monsters to get back a little mana when you can (i.e. when you're not too busy DPSing the monster with other spells).
The skilled discipline priests will weave in the occasional shield while DPSing.
If that seems negative, it's not my intent. I'm being heavily tongue-in-cheek. Blizzard has generally said that this kind of playstyle is viewed as optional. Of course, Evangelism, Atonement and Archangel now being baseline to discipline suggests that maybe it isn't. One resolution we could draw is that Blizzard's view is that you can successfully disc without using these things, but if you want to be truly optimal (i.e. higher skillcap), you'll weave all these in. On the other hand, maybe they're balancing it such that it truly is optional (whether you're wandering around old content or pushing the edge of progression) and they just didn't want to waste more talent and glyph slots, so they stuffed most of these abilities into the discipline baseline arsenal (Atonement, at one point in the beta, was a glyph and Archangel, until this latest build, was a talent).
Further, I don't dislike the whole "DPS for rewards and prizes while healing" idea. It does open up another alley for healers who want to change their style of game in a meaningful way. I now consider Evangelism/Archangel/Atonement to be part of my baseline discipline spec and I do use them (plus I'm amused by healing heroic dungeons purely by holy firing and smiting). I quite like the Telluric Currents dynamic on my shaman. Now I can put them together, like chocolate and peanut butter. Or maybe like bananas, peanut butter and bacon. I'm really not sure how it will actually play, because man...that's a lot of DPS spells to juggle. I still do cast healing spells when healing, you know. Well, not healing spells...shielding spells. Never mind.