I just thought I'd write a few quick thoughts about stats as they pertain to healers, and then a little chatter about stats as they pertain to discipline priests specifically. Because aren't disc priests about the most interesting people in the world? They drive fast cars, move in fast crowds, heal with spectacular style before burning out and dying of a cocaine overdose. (RIP BubblesTheCokeMonkey.)
Stats are generally considered to exist along a couple of axes (plural of axis, not axe): mana efficiency and throughput. Mana efficiency helps dictate how long your mana pool will sustain your healing activities and throughput dictates how much healing you can pour into any constrained period of time. Throughput is always crucial in any current content, while mana efficiency's importance varies with how far one is into the expansion.
The reason why mana efficiency's importance varies is fairly straightforward: spell costs are based on one's base mana (the mana pool you have with no gear equipped) while regeneration scales with your stats. The two tend to synch until you reach level cap, because each level raises your base mana and, therefore, your spell costs. Once you reach level cap, however, your advancement is no longer level-based but gear-based. Your base mana is locked in "forever" (that is, until the next expansion raises the level cap) but your stats keep increasing with better gear. This creates an asymmetrical dynamic and, as the expansion wears on, the mana pool and mana regen that your gear provides will begin to trivialize spell costs.
For now, mana efficiency tends to be useful, but already the use is variable. I find that the only normal raid encounter for which mana efficiency can matter is the Nefarian encounter in Blackwing Descent. Mana efficiency does matter to varying degrees in heroic raid encounters. However, next tier, mana efficiency is going to start fading fast as any kind of priority. Bear that in mind as we go through the stats.
Getting back to my original line of thought, I think that looking at mana efficiency and throughput as separate categories is imprecise. Certain stats have an element of each and each component needs to be kept in mind.
Intellect Is OP
On the World of Warcraft stat forums, all the other caster stats write long, angry posts about how intellect is unbalanced and overpowered. They're right, but tough luck. Blizzard has made a clear and conscious design decision that intellect should always be the #1 choice of any caster. Unless they reverse that decision, and it doesn't seem likely that they will*, you will always want intellect first, any time there's a choice. It provides mana efficiency, throughput and candy all at the same time.
*The reason I say that it's unlikely that Blizzard will ever reverse the decision to make intellect far and away the best caster stat is because it's a standard role-playing trope that there be a certain stat that is defining for a class (generally intellect for magic-users, strength for power-based weapon-users and agility for speed-based weapon-users). A game like WoW might try to subvert common tropes early on, to try to find a "new way," but when it decides to finally adhere to one late in its development (WoW is now about 6 years old, which is pretty far into a game's development), it generally means the design team decided it was a convention worth sticking with. The time for experimenting with that particular aspect is probably over.
It provides mana efficiency because your mana pool is directly increased by your intellect. Larger mana pools mean a larger buffer to cast from before you run dry, but if that's all they were, it wouldn't mean much. However, a number of mana regeneration tools operate by providing your a percentage of your total mana pool back periodically. That means, the larger your mana pool, the more mana you're going to get back over time when these mechanics are in play.
The throughput largely comes from the boost intellect gives the output of any damaging or healing spell known as spellpower. You get spellpower at a one-for-one rate from intellect, but each spell has its own modifier for what fraction of your spellpower gets added to the spell's output (known as the spellpower coefficient). This is added to every spell and is a pure throughput boost.
Finally, don't forget that intellect provides candy. And in a wholesome way, not dispensed from a scruffy stranger in a nondescript van. The form of this candy is a little bit of extra spell critical strike. It's not a massive amount, but the previous effects were enough to make intellect fantastic, so this is just gravy. Candy gravy. Mmm, food metaphor stew.
Spirit is a little bit like intellect. It's sort of the little sister to intellect's big sister. It tries to emulate intellect, and does a decent job, but falls crucially short. Just like every younger sibling.* You see, like intellect, spirit is both good for mana efficiency and throughput. However, here's the big difference: it can't do both at the same time. Intellect can.
*If you're a younger sibling that was traumatized by just now reading what the world has been telling you your entire life, please send all psychiatric bills to your parents, c/o a cold, cruel world.
So spirit provides mana regeneration in the very straightforward manner of giving you a constant mana-per-five-seconds rate of return. For most healers, the rate is halved in combat (holy priests, bless their mana-burning hearts, get a talent to increase it to 80% of their out-of-combat rate). For casters who weren't quite able to pass medical school, spirit provides nothing at all in combat. So, this component is easy and direct: more spirit, more mana provided over time during an encounter.
The less-appreciated aspect of spirit is that it can be converted into throughput. In a sense, this is semantics, because this isn't a separate power, but it's still an important observation: having more spirit, and therefore more mana regen, allows you to cast more mana-inefficient spells (but better throughput ones). The reason this is an important point to make is because healers often start reforging away from or otherwise dumping spirit too early. They've just become comfortable with their ability to last throughout encounters, so decide they should move toward more throughput stats. However, since they aren't at the point where mana is largely irrelevant, they suddenly find themselves in a tough encounter (either a new progression fight, or a fight they've done but isn't going well this time) and suddenly mana is leaking away alarmingly fast. If they had stuck with the spirit, though, they would have their comfortable regeneration rate now when they need it. And on fights where they don't need it, they can cast more fast, strong spells to convert the mana regen into throughput (in much the same way Einstein discovered that you can convert matter to energy).
Spirit is a swing stat. On fights where your mana pool is more than sufficient, you can make it do throughput tricks. On fights where you need mana, you can let it do its usual mana regeneration thing. Intellect gives both at the same time, spirit makes you choose.
The Eternal War Between Haste And Crit
Read ten different blogs dedicated to healing and you'll get...uh, well, two different opinions on which is better. I guess there's not a lot of position choices here. However, let me illuminate the parameters of the schism and you can make your own choice as an informed consumer of stats.
Both of these stats are pure throughput stats, in that neither provides a mana efficiency benefit (the way intellect and spirit do). However, that's not the whole story. One of these stats does impact mana efficiency, just not in the positive direction. Of course, even that comes with a little complexity. Almost like little people, these stats are complicated and multifaceted.
Haste does what you'd expect: it makes you a faster caster. This is manifested in three ways, two of which I'll mention now, one in a moment. The most obvious effect is reducing the cast times on spells that aren't instant. The less obvious but also important effect is to reduce the GCD (global cooldown) that limits how quickly you can do most abilities in succession. Reducing the GCD is important in pumping out instant-cast spells faster.
This obviously has the effect of raising throughput. Your heals per second are going to go up if the seconds consumed by casting go down for the same effect. However, it also has the effect of reducing your mana efficiency. To realize the throughput benefits of haste, you must cast more spells in the same amount of time and each spell still costs what it did before. Net result: your burn through your mana faster in order to raise your throughput.
However. There is an exception to this, and it's due to the third effect of haste. The third effect of haste is that it speeds up the ticking of your HoT (healing over time) spells. HoTs provide a certain amount of healing every "tick" over a fixed duration. The more haste you have, the more healing ticks you get in that duration. Therefore more haste means more throughput from each HoT, notionally. In practice, this is one of the most complicated things to math out, because you need to add a certain amount of haste for the HoT to speed up enough to get one more tick in before the duration ends (known as a "haste breakpoint")...and every HoT has different haste breakpoints. Still, there are online tools and tables that can help you figure out how much haste you need to reach the next breakpoint for any particular spell. The crucial thing to understand here is that this is an application of haste that increases throughput with no ill effects on mana efficiency. Your HoTs will still cost the same and run for as long before they need to be refreshed. They'll just heal for more (if you reach the breakpoints).
So let's now examine critical strike. Critical strike raises your chance of a healing spell cast to be a "critical heal," which means that its effect is multiplied. Right now, it's multiplied by 1.5x. With the coming patch 4.2, a critical heal will be multiplied by 2x. Critical strike does not share haste's downside. You don't have to spend any extra mana to realize the throughput benefits...some of your spells will simply heal for more, no muss, no fuss.
Critical strike, however, has its own dark side: it's random. It's the stuff of chaos, it swirled in the primordial universe before the gods brought order. This presents a problem if you like predictable throughput. You cannot count on critical strike throughput when you need a large heal, because most likely you will not get one on any specific cast. Further, you may get one when you're topping someone off and lose the benefits of it to overheal. This does de-value it to some extent, because you cannot account for it and tailor your heals appropriately to ensure none of it is wasted. You have to heal as though it's not there and let it pop up and surprise you here and there and hope that over the course of the fight, not much of it is wasted.
The added complexity that critical strike brings with it is that there are a number of talents that are activated by a critical strike heal. These talents boost the value of critical strike. They don't make critical strike any less random (nor should they, randomness is its nature!) but they do increase the rewards when you pop a critical.
So which is better? Well, from a philosophical standpoint, if you value more predictable throughput that you can plan for heal by heal, haste is better. If you value increasing your throughput without harming your mana efficiency, critical strike is for you. However, when you factor in the complexities of haste breakpoints for HoTs and talents that activate off critical strike, the question becomes a lot more difficult and is also quite class- and spec-dependent. It's a war that will rage in the cosmos for thousands of years.
Mastery: "I'm Here Too, Guys..."
The thing to understand about mastery is that it (quite intentionally, I assume) marries the benefits of critical strike and haste, thereby carrying the downsides of neither. Like haste, it is predictable...it carries none of the randomness of critical strike. Like critical strike, it provides its throughput with no ill effects on your mana efficiency...it does not force you to expend more mana to utilize it, like haste does. This description of mastery is true for every healing class.
That is not to say that it is equally valuable for every healing class. Mastery sounds a bit like spellpower in terms of it having all the positives of throughput and none of the drawbacks, but there's a crucial difference. Spellpower passively powers up every heal you cast. Mastery only powers up spells based on certain conditions, and those conditions vary by class and spec. That can drastically effect its value, so there's not a lot more to say about mastery that can apply generically.
Stats Get Hotter When Disciplined
So obviously I think discipline priests are about the sexiest and best-designed healers ever to grace God's green MMO. However, I'm a little biased: I love beautiful things.
All that said, here's a little objective evidence: every one of these stats is awesome for discipline priests. Sure, we may in certain situations want a bit more of this or that...but we don't hate any of the stats and they're all pretty great. The only itemization concerns we ever have is a general striving for balance, because we want everything. You'll rarely hear a discipline priest say "Yeah, I could really use that except it has critical strike" or "My tier pieces are terribly itemized for me" (though complaining about tier set bonuses? Gladly!).
So, discipline priests are in a good place. Intellect and spirit are obviously good for us (they're good for everyone, at least in this brief window of time where mana efficiency matters), we have a bunch of talents that activate from critical stike, it's always nice to be able to cast faster at need and our mastery boosts our many glittering shields (and any discipline priest who doesn't love bigger shields is a communist and a fascist at the same time. I don't think that's too harsh).
So, was this final section actually meant to be informative or simply a blatant advertisement for how awesome discipline priests are? Yes. After all, knowing that discipline priests are awesome is valuable information for everyone.