My catastrophic, and somewhat traumatic experience the other night on my Holy Pally was incredibly humbling. Normally in my 10 man Sunday night group, I'm running as shadow, which is something that previously I had never really done much in a raid. As someone who has ALWAYS raided solely as a healer, it has opened up an entirely new concept to me (don't you dare laugh, all you DPSers!) - the Rotation.
Sure, sure, I'd read about the theory behind it... making sure you have 5 stacks of shadowy thingie goodness, timing your Mind Flays with Mind Blast, etc etc etc... but knowing about it in theory, and performing it well in practice are two very different things (as my experience trying to heal as Disc has shown me!). Once I finally get my rotation down, however, I'm completely bored. The challenge is gone for me now, and I'm looking around for any emergencies that might require me to do a little backup healing...
But I digress. I really do have a point.
When I'm running holy, I'm johnny-on-the-spot. I am, by nature, a highly caffeinated, fidgety, jumpy person. I may not always have the most appropriate response to a stimulus, but respond I do - my reflexes are definitely there, sending limbs akimbo and random curse words spewing from my mouth. BUT AT LEAST I REACT QUICKLY. That's what's important, after all!
So therefore, I do well as a raid healer. There is very little rotation involved, and even with the small amount that Blizz has pushed on us through Serendipity, it's more of a nice bonus effect for something I would likely be casting anyway. Raid healing is generally more of a Reflex response.
Types of Damage and the Proper Response
There are many levels of predictability in raid healing, and they all require different amounts of preparation, and a different mix between rotation and reflex.
1) The Predictable Raid Wide Damage. E.G. XT's tantrum. You know when this damage will hit, and you know exactly whom it will hit. So for instance, for a Holy Priest, you can build up your Serendipity stacks beforehand to get a hasted Prayer of Healing off during the damage. Perhaps a Disc Priest will start pre-Bubbling people into safety before the inevitable tantrum.
2) Unpredictable Damage, Predictable Target. You might also simply call this spike damage on a tank. Think Patchwerk. You don't know the exact second the off tank will be hit with a Hateful Strike, but perhaps you know it will be the offtank who gets it, you know it will be happening often, and therefore you can just keep heals up on that target constantly. Usually, the best way to handle this type of thing is just steady healing, or keeping the potential target shielded, or buffered with HoTs. This isn't really that significantly different than the next type of damage, and they go hand in hand with one another...
3) Predictable Damage, Predictable Target. This is almost always the tank, and this is the damage that is healed best through rotation. The most optimal single target healers have solid, if somewhat more complex, healing rotations, and that is part of what makes MT Healing a bit more interesting than it might be otherwise. For instance, if you stick a Holy Priest on MT healing, they will spend most of their time working with the one rotation mechanic we have through Serendipity - Flash Healx3, Greater Heal. It's not that complex, but it is something - it is our only real rotation. But put a Pally Healer there, and there will be some funny business with a Sacred Shield and a judgment somewhere, some Flash of Light thingies... well you get the idea. (Obviously if I understood the rotation for Pallies better, I would not be so horrible at it)
4) Predictable Damage, Unpredictable Target. Think Slag Pot. Light bombs. Kologarn's fists. Frost Bombs. All of these mechanics are things you expect, but you cannot exactly prepare the target, because it could be anyone - so you simply have to respond quickly.
5) Unpredictable Damage, Unpredictable Target. Everyone screws up sometimes. Everyone steps in the fire on occasion (at least at first!). You can choose to ignore people who consistently do this, of course, but usually we just need to forgive and flash heal.
More than Simply Single Target and AoE Healing
Any time you have an unpredictable target for damage, powerful, reflexive single target healing is needed. You don't have to be an AoE healer to pick this up - and this is one of the reasons that so many Disc priests actually do well on raid healing. Their ability to respond quickly to bombs and fight mechanics makes them valuable to a raid, even if they aren't focusing on the tank. But, let's not forget that Pallies as well have one of the best single target burst heals available - Holy Shock is instant, with a reasonably short cooldown.
The idea that those who heal the Main Tank are just spam healing is not accurate. It is possible to simply spam, but from what I'm beginning to understand about the different healing classes, if you are spamming the tank, you are either not playing your class properly, or you are not really built to focus your heals on one target. A good Main Tank Healer weaves in their abilities through a solid rotation to make the most use out of their spells, talents, and cooldowns.
Raid Healers are built for reflex and rapid response. AoE heals are also important in raid healing of course, but I think sometimes we focus too much on the power of one or two spells, be it Wild Growth, Chain Heal, or Circle of Healing, and forget to note the synergy between our spells and talents. Main Tank healers need to know how to build a strong rotation, whereas raid healers need to know which abilities will work best in a split second situation, with an eye on cooldowns and procs. Both are challenging, but in different ways, in different scenarios, and for different people.