Yesterday, my priest ventured into Karazhan after having not been played at all for ... oh gosh, a long long time.
She doesn't have any of her stuff enchanted, except for a glyph and runic spellthread on her kickass pants. I have all the Hallowed gear (except the pants, but that's okay.). But not my primal mooncloth set (working on it!)
Still, with a potion of healing power and golden fishsticks, she's at 1350 bonus healing, 400 something mp5 resting and something in the 100's while casting.
The only time I ran out of mana was during the Maiden fight, and that's because I died and soulstone doesn't restore a heck of a lot of mana.
I love my priest. :)
Anyway, the point of this post is that different healer types do different things well. You have your paladin healer, your priest healer, your druid healer and your shaman healer. Each of them play particular party roles.
The paladin is, from everything I've seen and read, King (or Queen) of single target healing. Low mana, high value heals. What they lack is any form of a HoT or any multiple-member-heals. But they are like the energizer bunny. They just keep going and going and going and going...
The priest is undoubtedly the Empress of healing. We have a HoT. We have low mana-moderate value heals, we have moderate mana-high value heals. We have party-heals. We have binding-heal. We have prayer of mending. Depending on particulars, we also have have circle of healing and lightwell. What we lack are FAST heals. Flash heal, IMHO, isn't as mana-efficient as me just anticipating and pre-casting a larger heal or greater heal. The only time I flash heal is when I NEED to give someone just that little bit of health ASAP. AOE'ers being hit whilst AOEing, for example. But I notice my mana drops a lot faster if I'm spamming flash heal than if I'm anticipating a greater heal or regular heal.
The druid is .. okay, I've run out of terms that are applicable. The druid is AWESOME at instant cast HoT's. They have the most HoT's, that they can turn into a big ass heal by using Swiftmend. And while they can main heal something like Karazhan on the main tank, I think utility wise, they make far better party-toppers than they do main-tank healers. Need a little rejuv here a little there, how about a lifebloom? Not that they can't do it, it just is a better utility of resources, IMNSHO*.
Shaman healers are good at .. not healing. Earth Shield, Health totems, chain healing, that silly little - spam little heals to get a big heal - whatever it is they do. Honestly, shaman healing is the one I'm most clueless about. They seem to fall in the middle of the road though. Big heals, little heals, fast heals. Owaru, I'm looking at you buddy. Fill me in.
So, my priest is healing the offtank in Karazhan. This is fine. Except I didn't do much healing! Not merely because the tank had decent gear so wasn't taking much damage when he actually WAS tanking trash, but also because the same was true for the healer who was on the MT and the healer who was doing party heals. All of us were so READY TO HEAL!!!! that we were healing each others people. I know I saw a few HoT's that I didn't toss on my target on him. (I'm looking at you, druid! MY TARGET, MINE! HEALS OFF! ... (get it, hands off, heals off? Oh, nevermind!)).
I spent a vast majority of my time on trash mobs ... DPSing. I felt kind of guilty. For a while, I was pacing the OT on my DPS meter. Then I decided I should get down and serious and only healed... except for the occasional smite, mind blast and shadow word.
I don't know how other healers do it, but I tend to toss a renew and PoM on my target at the start of the fight and keep one up. Then I usually cast Rank 3 or Rank 4 Heal. Not Greater Heal. Heal. If I'm falling behind (not usually, but depends on the tank), I switch to mid-level Greater Heals. If a DPS class is getting whacked occasionally, I toss a renew on them and if the tank is doing their job, that's all I usually need to do. This obviously doesn't hold true if it's an AoE pull or something similar.
Like I mentioned earlier, the lowest on mana I ever got was 6500 mana on any trash pull, with me healing AND DPSing. (I have only 9something total mana)
For my shadowfiend... him I toss out when I get about 1/4-1/3 down on mana. This way if it's a long fight, he MAY be ready by the time I need him again. (Tip for Curator fight for priests, wait for his second evocate, if you can, before you sic your fiend on him. Double-damage means your mana bar jumps up! and double-damage to him, as well.)
To the warlock, I apologize for your death in the Maiden fight. I was hitting the wrong button and going 'Why the hell is her Holy Fire not going away?!'. I was hitting the RIGHT button for the entire rest of the evening, but for some reason, I didn't do it that time. In my defense, the paladin healer also didn't cleanse you in time, so it isn't entirely my fault! Sowwy.
(* In My Not So Humble Opinion)