I did two instances as disc. I'm gonna get me a snack and write up my experiences; this is gonna be a long one, even by my standards, as there's a LOT to report. Note that this is disc-only for now. Holy will be similar, but I'm not trying that yet until I'm more comfortable with the whole Cata way of healing, as literally EVERYTHING has changed. But anyway...
Alright, lets start off with a small explanation of where I'm currently at. My priest is level 83, and has 48K hit points (54K with Fortitude) and a mana pool of 54K unbuffed - in one of the groups we had a mage and druid, who'se buffs put me up to 60K. I'm using
this spec with the normal WOTLK disc glyphs (Flash Heal, PW:S and Penance). I'm on roughly 4500 spellpower unbuffed, 4850 with Inner Fire. I've got 10 mastery which increases the potency of all my absorbs by 25%.
First of all... Holy spaghetti meatsauce, healing has changed. I hope you guys remember the Vanilla times, because that's what this is closest too. I'm not ashamed to say we had at least two or three wipes in my first dungeon that were entirely my fault due to me running OOM or not keeping up with healing. I will take a little bit of the blame away though - I was using the standard Blizzard interface, which, when scaled down, makes it impossible to keep track of health bars. I flipped it over to the standard Blizzard raiding interface, which is basically a pretty, drag-able version of grid, somewhere during the first instance, and that improved a big lot of my healing.
Imagine every spell a disc priest has in his ******** and then imagine that they would all suddenly have a COMPLETELY different function and application. That's how I felt. I did two instances and it indeed took me both of them to get fully acquinted with the class again. To start with, mana is HUGE. I found myself drinking
after every pull. And I should have relatively good gear, too (IcC25 heroic to a level 82 instance).
Because mana is so huge, your go-to spell is Heal. If you can use Heal, you WILL use Heal, or you will be OOM within the next twenty odd seconds. When I say everything has changed, I mean it. You no longer cast PW:S on everyone. ProM constantly on cooldown? Forget it. Renew on the tank? You don't have the mana for it. Flash Heal to quickly heal up a DPS? LOL. Get this: Heal is 623 mana. Penance is 1476, Flash heal
THREE THOUSAND, Prayer of mending about 1.9K, PW:S 2K, Renew 2.2K, Prayer of healing nearly 4K, Greater 3.4K and Binding Heal 3K. I'll say it again: Heal is the backbone of your ******** If you can use Heal, you WILL use Heal. You will also use all your cooldowns on any given boss fight, and be extremely happy Inner Focus' CD went down to 45 seconds. You'll hate that it no longer affects anything but your three "Heals" (Flash/Greater/Normal) and PoH, though.
Because of all this, the roles of every spell in your ******* have changed, and you really need to think wisely about when to apply them. Prayer of Mending is simply no longer worth it unless there's consistant AOE damage going around. Power Word: Shield, even as disc, is more of an emergency or pre-pull thing. No worries Teacake, there's still plenty of it around (especially with the new Strenght of Soul talent), but rather than it being 80% of all your casts, it'll be more like 10%, with Heal taking the 80% spot.
And heck, even Penance doesn't live up to Heal's obscene mana efficiency - its instant first tick is golden now, and because of the longer cooldown, Penance really is more of an emergency heal than anything else. A kind of "There's a big blow coming and I need to fill the tank up" spell. Flash Heal has become a spell you never use outside of real "This guy NEEDS to be healed NOW or he's dead" emergencies, though it is partially replaced by Penance as it is now.
Not even Renew escapes the "Efficient spell? SOD THAT, YOU USE HEAL NOW!" wrath Blizzard has unleashed. With it having a 2.2K mana cost it's something you use to either top DPS off if you can't spare more than a GCD off the tank, or to help out on the tank when you know a tank-healing intensive phase is coming up. Note though, that phases that require "intensive tank healing" aren't even comparable to how they are in wrath. Intensive tank healing means that the tank "will slowly start dying over a period of about 10 seconds, and that you need to be prepared to chain cast Heal on him to keep him alive". If you're not, and you won't be, since your entire party will hang around 50% hp for the larger part of most fights, you need to Penance to buy yourself some time and get some Shields or Renews up on those DPS.
Greater Heal is something you use ocassionaly, but it has largely the same role as Heal - there's a really thin line between when you use which one. I myself tend to use Greater Heal if I'm spamming Heal and the tank's health isn't going up or down. Binding Heal sees more use, though I think it's a shame not even that spell is mana-efficient, as it fits right in with the whole "you need to use the most efficient heal possible" theme that's been going on - it doesn't feel rewarding when you get off the full effect from a critting Binding Heal and you still lose half your mana pool. Prayer of Healing is still your go-to group heal, but only if you know you will actually heal all five people with it for the full amount. It's not spammable though, and not really reliable on getting your entire group topped off.
I also just want to take a moment to say that PW:B is now officially the worst cooldown I have ever seen. At one point in a boss fight, there were a lot of adds involved, so I threw it down on our (fairly) clumped up group to lessen some of the damage coming in. I lost 7K mana of the 10K I had left, we killed the adds within about 2 seconds, moved out of the bubble, and that was basically it. It needs to be twice as large, and even then it'll probably still suck. Even if I throw it on the tank and get full mileage (50K absorb) out of it, it's not even near as effective as 50K heals worth of... Heals (the spell) would be. As it is now, it's situational beyond belief. You'll save it for the same situations where you use Holy Nova in WOTLK. Except Holy Nova doesn't have a 3 minute cooldown.
Last but not least, there's the whole Smite/Atonement/Archangel thing. I've not yet *really* found a place to fit this in, as you generally spend the whole fight trying to fill up everybody who'se at 50%, but I have seen some opportunities where it could be used. The thing is that Archangel's mana regeneration cancels out the Smite cost, which basically means that if you have ~7 seconds just before an intensive phase, you could do the Smite Trick and earn yourself so more efficient healing for the phase to come.
All in all, like I said at the start of my wall of text, healing as it is now mostly resembles the way it was in Vanilla. It's just that cancelcasting has been replaced by chain-casting Heal, as you suddenly have one sickly efficient spell in your ******** To note; chain casting Heal basically breaks you even in mana - it's mana cost is virtually the same as you regenerate.
I will also mention that as it is now, definitely in raids, survivability WILL be a huge factor for DPS classes, and I might or might not expect my rogues to keep up Recuperate. I will
also mention that in this environment, Lightwell should be kickass if summoned at the right spot. But more about that once I get to give Holy a whirl.