Let’s start from the beginning. (I’m told that it’s a very good place to start.)
Right now, the conventional wisdom is that Discipline priests are for single-target healing and should stack crit (for Divine Aegis and big heals), and that Holy priests are for multiple-target healing (and should stack Spirit for the spellpower bonus and mana regen). I’ll say up front that this is a perfectly valid way to play the class. I don’t think that either of these assessments are untrue; Discipline priests are strong single-target healers, and Holy priests are strong multiple-target healers.
Just because Discipline priests can be strong single-target healers doesn’t mean they can only be strong single-target healers. WOW itself has an excellent example of this kind of dichotomy: just because Feral druids can tank doesn’t mean that’s all they’re good for. I see another approach to Discipline healing, which focuses less on talents like Divine Aegis and more on talents like Borrowed Time and Renewed Hope.
Think about the distinction between fast and slow weapons in WOW for a moment. At a given level of DPS, slow weapons hit harder but less often; fast weapons hit far more often, but for lower amounts. With proc-per-minute weapon buffs, a fast weapon has a much lower chance per strike to activate its effect than a slow weapon does, but since the fast weapon is hitting more often, it evens out. Each has its advantages and disadvantages.
So it is with Discipline healing. The conventional-wisdom method is equivalent to a slow weapon: it doesn’t cast very fast, but it heals for a lot. However, talents like Borrowed Time, Improved Power Word: Shield, and Renewed Hope, and the Glyph of Power Word: Shield, make a fast-but-weaker option both viable and desirable. Instead of stacking crit like CW Disc priests do to increase their chances of getting Divine Aegis off, fast-but-weak Disc priests stack haste. These priests don’t heal for as much as their crit-heavy brothers, but they are much more agile in any environment, and specialize in hit-and-run healing.
Note that this is a question of approach, of philosophy, not of gear. It’s actually improbably easy to reach the soft haste cap with this philosophy; assuming that you have Borrowed Time and Enlightenment, it only takes 66 Haste Rating to reach the 33% haste cap after you’ve cast Power Word: Shield. This cap is based on reducing the 1.5s global cooldown to 1.0s, which is as far as it will go; at 33% haste, multiply your spell’s cast time by 0.67 (1-0.33) to get the actual cast time. This reduces the cast time of Flash Heal to 1s, the channel time of Penance to 1.3s (a tick goes off at 0s, 0.67s, and 1.34s), and the cast times of Greater Heal and Prayer of Healing to 2s. Further Haste Rating will increase your haste (and lower your casting time) even more, and it’s possible to reap some advantage from this, but more haste has a diminishing effect (because of the
The idea behind this approach, therefore, is that you’ll be casting Power Word: Shield at just about every opportunity. This will both ensure that Renewed Hope stays up on your targets (both the -3% damage buff and the +4% crit chance) and allow you to cast your heals as quickly as possible. You’ll also do a lot of mitigation of damage, and since you’ll be keeping the Weakened Soul debuff up on a lot of players at once, you’ll also have an increased chance to proc Divine Aegis when you do have to heal someone.
There are, of course, drawbacks to this approach. The first and most obvious is that it goes through a lot of mana. This can be mitigated by a high Intellect and mana regen rate, and even without those advantages, I had little trouble in 10-man fights when I tested this approach last night. Another objection at first glance is that by by relying on casting Power Word: Shield before you cast your heal, you’re effectively adding to your cast time (the half-second shaved off your Flash Heal is offset by the 1-second GCD from PW:S). My only response to this is that it’s better to be casting PW:S almost constantly. By doing that, you’re keeping yourself in Borrowed Time for when you do need to heal somebody.
I admit that this is an approach that feels strange to a lot of healers, myself included; spell haste is for caster DPS, and PW:S is for emergencies! I think that’s a Burning-Crusade, Holy-style healing mindset, though, and getting used to Discipline as a healing spec means getting rid of some of our preconceptions about what it means to heal as a priest. We can start with allowing for the possibility that PW:S and our caster stats can be used differently than we’re used to.
(Yeah, I’m kind of bad at conclusions.)