Okay, back to writing about healing (heck, back to writing anything about anything).
The biggest misconception I see about healing regards spirit. Spirit, in all honesty, is not a very complicated thing in World of Warcraft (it's arguably complicated in real life). It quantifiably increases your mana regeneration, which allows you to cast more spells. We can tabulate how much extra mana we'll get from spirit (thus, quantifiable) and we can quantify how much healing power that mana provides us.
I emphasized that second part because it seems to get lost in many/most/all healer discussions about spirit. Spirit has become a very mystical* concept in the healer world, for reasons that aren't entirely clear in this increasingly mathed-out game.
*There's some irony in decrying something as having "mystical" standing in a game about magic and dragons. But I digress.
Ask about how much spirit one should have as a healer on a major healing forum and the answer you'll invariably get is, "Enough to feel comfortable then start stacking 'throughput' stats." This is wrong on a lot of levels, but the root is that people just aren't thinking about spirit the right way.
Now, I realize that asking "How much should I have?" in a public forum is always going to be a meaningless question, for any stat, if you're looking for a number. However, we can give non-numeric answers that help the asker (or ourselves) come to good results. If someone asked "How much critical strike [or haste or mastery] should I have," I can guarantee you that the most popular answer wouldn't be "Enough to feel comfortable."
"Comfort" is such a vague, nebulous word that we're already starting off on the wrong foot. Healers seem to mean, by "comfort" with spirit, not noticing themselves running out of mana (OOM) very commonly in whatever content they're competing in. That's a really poor metric, because mana usage is fluid...not only do different encounters have different demands, but different playstyles can use mana differently. There isn't a static amount (or even a modest range) of spirit that means you're incapable of running out of mana...this isn't defense capping (for those of you who go back at least to Wrath) or full combat table coverage (for those of you who go back at least to Cataclysm). Even on a per-person basis, your spirit needs change and they should always be changing based on the situation.
Healers have, for some reason, come to regard spirit as a force outside of the optimization game, something not well understood and best left up to "feel" or one's gut. Does your current spirit total feel good? Then you have enough! Doesn't feel good? Get more! Can't say whether it feels good or bad because there's really no frame of reference and the question is a little silly? Um, well... [end transmission]
So What Is Spirit?
Spirit is a throughput stat like critical strike, mastery and haste with one difference: rather than empowering your spells, it empowers you to cast spells. This difference seems to throw a lot of people for a loop, but it's really not a huge difference.
After all, we can quantify this. Spirit, at level 90, gives you this much mana regen (per five seconds): 6,000 + (0.56435)Spirit. We can multiply that by 12 to get how much mana regen per minutes. Let's say you have 12,000 spirit. That formula yields 12,772 mana per 5 seconds (MP5), which is 153,266 mana per minute...approximately half your mana bar per minute.
What can you cast with 153,266 mana? Or, to be more fair, what can you cast with 81,266 mana, since 6000 MP5 comes for "free" per the "6000" constant in the formula, which equates to 72,000 mana per minute if you had no spirit at all. Over a 6 minute encounter, your 12,000 spirit is netting you 487,598 mana.
Is that worth more than investing those 12,000 points of stat budget into things like critical strike, haste or mastery? That's a much more complicated question, of course, and it depends greatly on class, but it's one that can be investigated.
You can do the same exercise on a smaller scale, like whether cutting down from 12,000 spirit to 11,000 spirit is a good idea. Or whether a spirit trinket is better than a haste trinket. You can calculate the numbers and then evaluate what you'd spend that mana on. That gives you a much firmer perspective on how much spirit you need than "how it feels."
What Can We Spend Mana On?
Okay, yes, heals, of course (unless you're a discipline priest, then DPS spells). But which healing spells? Ignore your spells that come with cooldowns attached. If those spells are good ones, you'll be casting them pretty much on cooldown and more mana won't allow you to cast them more because the cooldown prevents you. Mana doesn't give you more Penances or Swiftmends. What it will give you is more of your spells that have no cooldown, your "spammable" spells. These are mana-limited...you cast as many of them as your mana allows, around your cooldown-limited spells.
So we're talking about spells like Rejuvenation, Prayer of Healing, Chain Heal, etc. These are the spells that you should check the mana cost on, compare it to the mana you'll get from spirit and from that calculate the approximate throughput you're adding.
If you have a World of Logs record of your performance, the comparisons become reasonably straight forward. For example, you can figure out how much critical strike percentage you'd get from the same amount of stat budget and multiply your non-overhealing output by that added percentage and compare that with the added healing you'd get from the X extra Rejuvenations you could have cast, as an example.
Of course, that does introduce a tricky element: over-healing. If you critical strike more (or cast faster due to haste, or just hit harder with your heals through mastery) some of that extra healing will likely be pushed into overhealing. This might actually be a benefit to more spirit...since you control where and when the extra casts go, presumably your decision-making will bias those spells against being overhealing (though with other healers in the mix, this is far from certain).
The point is not that you can come up with clear-cut certainty as to which will give you more bang for the buck, spirit or another secondary stat. The point is that you do not need to relegate this comparison to the realm of feelings and intuition-elves. You can sit down and take a more systematic look at it.
You neither want to blindly stack spirit, nor blindly cut spirit. You don't want to think to yourself, "I don't run OOM, so clearly I must cut spirit." You want to consider what spirit nets you (not mana, the healing output of the extra spells you're able to cast) versus the healing output that the other secondary stats net you. Logical thinking versus magical thinking. Spirit isn't weird and magical...it's concrete and it gives you more healing spells to cast. Nothing hard to understand or calculate there.
If I were to boil this down to a principle (beyond "investigate what everything nets you"), it would be this: If the spammable spells your class affords you are powerful, you're probably better off with more spirit. If the spammable spells your class affords you are somewhat lackluster, then you're probably better off with less spirit (other secondaries will allow your cooldown-limited spells, which you rely disproportionately on, to hit significantly harder).
That said, don't let anyone tell you that spirit is touchy-feely. You don't stack it til you feel comfortable. You stack as much of it as you can effectively use. If you have strong spammable spells and don't come anywhere close to running out of mana, that may just be a sign that you should cast more (and more expensive spells).
Hey, wow, I just wrote a blog post.