If you enumerate our protection spells, there aren't that many of them - Inner Fire and Power Word: Shield are the notable ones, in terms of spells that are available to any spec. Still, those are powerful skills, and we can't forget that we have some great heals too, in any spec.
Discipline - this is a healing tree that emphasizes survival and single-target healing. Discipline specializes in damage absorption and prevention rather than restoring health, although they are very capable in that department too.
As of Wrath, I'd say they finally nailed this goal. Between Pain Suppression, Grace, Divine Aegis, and a greatly enhanced PW:S, they certainly have the "damage absorption and prevention" down, and with Penance, as well as various talents buffing Greater Heal and Flash Heal, they do indeed turn out to be "very capable" at straight-up healing of damage. The upcoming changes in 3.1 are going to bring even more absorption ability with the new Power Word: Barrier (a group PW:S).
I've said it a few times before, but I don't mind saying it again: I'm really glad they finally made Discipline a viable alternative for raid healing. It's often felt like the grab-bag tree before (or, at times, the PvP tree), and now it has a clear purpose that it achieves quite well. Great job here, devs.
Holy - this is a healing tree that has a spell suited for every situation. Holy priests can heal groups, heal individuals, heal tanks, or do all three in a single encounter. They are better at group healing than Discipline priests, but don't have as many damage prevention capabilities.
This has always been the case, really. There is a very fine line between "a spell suited for every situation" and "jack of all trades, master of none," but I think we're on the right side of that line at the moment. I really enjoy the versatility that being Holy brings, and I do feel like I can tank-heal, group-heal, or spot heal just fine, thank you (with tank healing probably being the weakest of the three areas, due to the demise of downranking). Our group healing took a big hit in 3.0.8 with the CoH nerf, but it looks like we're getting buffs to CoH, PoH, and Holy Nova in 3.1, so we should be doing pretty well again soon.
The change to Serendipity that was announced for 3.1 is interesting. I really like Serendipity's current functionality, although on the other hand I almost never go OOM. The new mechanic ("now reduces the cast time of Greater Heal and Prayer of Healing when Binding Heal or Flash Heal are cast") should inspire us to shake up our casting orders a bit. I need to see the numbers to know how much of an effect this is going to have; if I was guessing, I'd say something like 0.2 sec per cast, stacks up to 5 times. It is currently unknown whether the new Serendipity effect replaces the current effect, or adds to it (i.e. whether we'll still get mana back on overheals in 3.1).
Shadow - this tree sacrifices healing for damage. Shadow priests can restore health and mana to their group while casting their spells. Like the Shadow damage of warlocks, Shadow priest damage tends towards drains and damage over time spells.
Shadow is doing great right now. They don't have unique mana battery capabilities anymore, but in exchange they get top-tier DPS (or close enough). Vampiric Embrace doesn't feel as significant as it used to either, but that's fine, again - yay DPS. I don't know about the drains, though - the main Shadow nukes are Mind Flay and Mind Blast, neither of which drains anything.
Overall this is an excellent document as a look into what Blizzard is trying to do with the Priest class. I only wish each class had something like this. It's true that, as Ghostcrawler said, players may try to use texts like this to "lawyer" the devs into making changes ("I'm supposed to be the master of AE damage" is the example GC gives, referring I suspect to Mages). But I think that's a small price to pay for this sort of communication with the community.
So what do you think about Blizz's idea for what a Priest is? And do you think they've reached it?