A rogue in my guild is working on making a competent healing priest alt. He asked me where the priest spreadsheet is. Like how you actually know for certain, the way a rogue can just say "Well, this sword is just flat-out better than that one because I put them both in the rogue dps spreadsheet and the answer is: the new one yields +10 dps." I answered, "Ha ha."
Healing is much harder to diagnose than tanking or dps is. I mean, trivially the question for healing is: Did we wipe? If yes, the problem might be healing, or it might be lack of execution or understanding of the event. If no, then you did fine. That's it. Bonus points for nobody dying.
When you're a dps'er, every choice regarding gear and talents and so on can be boiled down to one question (two if you're advanced):
- Simple: Am I personally doing more damage?
- Advanced: Am I making the group/raid's total damage output higher?
For each event, the raid has X time to do Y damage with Z constraints, now get to it. You can run Recount or Recap or any number of other tools to diagnose damage. It's trivial. You put on a new piece of gear, go raid, and then say,
"Well, I thought bonus crit rating this would help, but my miss rate went up by 2% and I did less overall damage. Guess I'm back to this weaker looking +hit rating blue."
As far as research, a class's dps is broken down into a format similar to this: If you are build A, stack stat/rating B until Bmax, then stack C stat/rating infinitely. For frost mages it's "stack spell hit until the spell cap, then spell haste and spell damage infinitely".
Tanking is harder than that. You're trying to both keep aggro and not die. Keeping aggro is about generating threat, which also has particular gear choices, but more or less works like doing damage (stack hit rating, expertise, spell hit, shield block, attack power, and shield block value in various amounts depending on which kind of tank you are). Not dying involves a few variables like stamina and avoidance, as well as preventing as much of the spike damage as you can. There are tradeoffs to be made, but you have a maximum health total that's easy to see, an avoidance rate that's easy to compute. You know you're doing the right amount of threat if your raid doesn't have to hold back on damage. You balance accordingly.
Healing? Like tanking, the suite of stats changes between classes–some mix of healing, mp5, spirit, intellect, and spell crit. But the act of healing depends entirely on each particular event. Your tank might get crushed, get hit normally, block, or dodge. Everyone in your raid might take steady, constant damage. Your tank might never drink potions or use healthstones. Your dps might get cleaved by the boss for standing in the wrong place. Your job is not just to heal the tank, it's to patch the mistakes in mid-event.
Also, two different attempts on the same boss with the same raid can go completely differently just due to the inherent randomness of some events. Think of Ilhoof in Kara. If a healer gets sacrificed, it's extremely tough on the remaining healers for those ten seconds. If not, that's nine minutes of (healing) pleasure! Switching one piece of gear in that event, and most others… you can't actually tell if it matters. You just know that a higher value in your key stats is better.
On top of all of that, in raids you also must instantly and silently adjust to the healing style of the people around you so that you're not wasting group efforts. That ability to adjust is the whole essence of raid healing–it can't be quantified and it doesn't show up on your character sheet. If I was going to land a slow, mana-efficient heal for 5000 and someone else throws in a mana-inefficient 2000 heal, causing me to overheal by 2000 and waste that time and mana, who's fault is it? (Trick question, it doesn't matter. That's wasted mana on someone's part, and if you don't work it out, the raid will wipe at some point due to your collective waste of mana.)
So given how nebulous and random and instinctive healing is, how can you tell that you're doing better by swapping one item for another? You can't tell, because you can't run tests outside of combat. Your heals might hit for a tiny bit less or more, but the actual trade off between 6 mp5 and 22 healing? Can't tell. Your playstyle influences your gear selection more than other roles. Damage dealers just do damage, and every piece of gear they have is about doing more damage. There's no opinion as to whether 900 damage is more than 800 damage. Tanks want to survive and generate threat, that's their balance.
A healer who loves mana regeneration is going to cast weaker heals continuously, and make it work. A healer who loves larger heals is going to try to time their larger heals correctly so they aren't wasting mana, and make it work. The final grade is: Did we win? Yes. Great, your gear and spec is correct for that event!
Most of us just learn the stats that are necessary for our build (for example, spell crit plus healing plus mp5 for a holy paladin), and try to get decent levels of all of them.
So unfortunately, there is no healing spreadsheet. This leads to the huge learning curve in learning to heal (and tank), but that's another article.