It's a fact, though, that which stat priority is correct for you depends entirely on your circumstances. What is your healing behavior? Who are the other healers in your raid comp and how do they prioritize stats? These things will ultimately determine your secondary stat priority.
I plan to put together a series of articles that should help everyone understand this issue well enough to choose for themselves. All the comparisons I make in these posts will be based on simulations, which you'll get to look at. The goal will be for earlier posts to be as general as possible, easy for casual players to understand. Later on I'll get into details.
Top disc priests on WoL use priority X, so should I!
This is often the reasoning people use to advocate pure crit stacking, because right now crit stacked priests parse highest. The trouble is, WoL can't show you the big difficulty with crit.
Be very careful with this line of reasoning. At this point in progression, ranked healers on WoL shouldn't be something to compare yourself against. If you look through healer rankings, you'll notice that their HPS is insanely high, probably much higher than you can achieve with the gear you're currently wearing. Also, there are only two ways to get a ranked parse.
The first way you can rank for hps is to be in a bad group. Somehow you don't have a resistance aura. People are taking excessive damage. Your other healers just aren't very good, so you're carrying heals. You earn a ranking, but the execution was horribly bad. It's just not worthwhile to try learning anything from a log like this.
The second way you can rank is to heal fights with fewer healers. For instance, if your guild were progressed to 5/7 HM you might be able to 2-heal normal mode Majordomo. This would be a legitimate way to earn a ranking, but at that point in progression those 2 healers wouldn't be gearing around that fight anymore. If some disc priest 2-healed normal mode Majordomo, you won't be able to take his gearing out of context. He's not even tuning for that encounter, and his gear level is probably much higher than yours.
Each secondary stat has basic benefits and drawbacks.
I compare everything to haste stacking, because that seems to be the most common idea in the community.
spike capability: By spike capability I am only referring to your max-throughput cast sequence. For disc priests raid healing this should be PW:S on rapture, then PoH spam. Haste is far and away the best stat for burst healing.A haste stacked priest can burst AoE heals 4% higher than a crit stacked priest, and 8% higher than a mastery stacked priest.
Is spike capability important? Sometimes. Is it the most important thing? Only if there is some specific mechanic that causes wipes for your raid group. For instance, if your group looks pretty unhealthy after ignition on alysrazor, or if the sudden push Rhyolith does into phase 2 is too much for you to keep up with, you probably need a little more haste.
Efficiency: For efficiency I am referring to the total amount of healing you can do during a typical fight. If you can improve your overall efficiency, you'll be able to fire off more AoE heals over the course of a long fight, and they'll be larger, too. Even if your spike capability suffers a little, you'll do more healing overall and your HPS will be higher at the end of the encounter.
In terms of efficiency, a crit stacked disc priest can get 15% more total healing over the course of a typical fight than a haste stacked priest. That's like jumping from 16k hps to 18.5k hps! Remember what I said about world of logs, though: just because someone is able to pull this off doesn't mean everyone should jump right away to stack crit. Raidhealing with crit is less stable. If you're raid healing with a high crit, you'll run into a number of difficulties. You might cause wipes, or you might see yourself spot healing a lot more - which would mean you're not actually getting any benefit from all that crit.
It's a subject that takes more than one paragraph to discuss, so crit for raid healers gets its own article. That will come soon!
On the other hand, a mastery stacked priest can get about 9% more sustained healing than a haste stacked priest. Mastery is also far more stable than crit, so from an efficiency standpoint mastery stacking is a reasonable idea. Remember, though, that stacking mastery hurts your burst potential.
Stability: Healing with high haste/mst and low crit is very stable. Healing with high crit is unstable. I choose not to refer to RNG for crit, because when you're AoE healing, penance/PoH will get many ticks over the course of the fight. During a typical beth'tilac, for example, I can expect my PoH to hit 300-400 times. The number of crits I get will be extremely consistent, because the spell hits so often.
You can count on crit to give you the amount of extra throughput you were expecting. This isn't the real problem that people discuss as 'crit RNG'. The real problem is: who gets the crit? It's never the person that needed one. PoH from a crit stacked priest will be extremely uneven, causing some people to drift towards high health while other people get dangerously low:
[I need a screenshot to put here. I'll try to fill this spot in the next couple days]
The only way you can escape this problem is with spot heals and smart heals. As a disc priest on my own, if I'm pumping max AoE heals, I only get one efficient spot heal (PW:S) every 12 seconds! Penance is a HPS loss, and an efficiency loss if I use it on anyone but the tank. Basically, you have to rely on your other healers to make up for your uneven PoH. Is that reasonable? Maybe, maybe not. Druids and shamans have pretty strong spot heals / smart heals. It depends on your group.
With MST or HST stacking, this problem simply doesn't exist:
We're looking pretty safe from that next scythe right now...
All my numbers come from two series of simulations. I assume the priest has 3800 secondary stats and that he can reallocate these however he wishes. Then I've run each sim at every possible combination of haste/crit/mastery, in increments of 200.
The first sim is a basic look at max aoe burst potential. It's a seven minute session of PoH spam with PW:S for rapture. No cooldowns get used at all, but random procs from enchants and items are allowed to occur. The priest has infinite mana. You can see it here.
The second sim attempts to model my actual healing behavior. Again, it's a seven minute session but this time it follows the most efficient healing priority. This priest uses his cooldowns in a semi-intelligent manner and has limited mana. This simulation should give a basic idea of actual stat values when efficiency matters. You can see it here.
Simulations are imperfect, so please do scrutinize my settings and give me feedback.
Summary & conclusion
So, how should we prioritize secondary stats? There's still no obvious answer. Crit is definitely amazing for sustained throughput, but it has its own drawbacks that we haven't explored fully enough. Mastery helps for sustained throughput without crit's stability problem, but its return per point is fairly poor. Haste is very strong (good for triage!) but unlike healers with hots, for disc more haste doesn't actually let you heal more. Unless, of course, you are ending fights with extra mana.
Don't let me downtalk haste, though. Triage is extremely important - being able to spike up farther at unexpected moments to react to things going wrong is indispensible. Preventing deaths has nothing to do with efficiency. Whether or not it raises your HPS, haste helps you do exactly that. It's always useful, it just has poor efficiency.
To finish answering the question, though, we have to look at different healing behaviors. We also have to explore the difficulties crit causes in a little more detail.
More to follow,
About me: I'm a repair electrician. I repair industrial machinery and write reports about the kinds of failures I run into. In this way I have some training to think like an engineer. So, this stuff probably reads like a technical manual! I tried to make it understandable, though. Comments? Criticism? Let me have it.