I have to admit it: I'm jealous of discipline priests. Not because their spec is sometimes a raid leader's preferred choice in PvE; that's all part of the normal up and down of class balancing. But because discipline is still considered to be the only viable PvP healing spec for priests.
I didn't mind the situation back when disc was designed for PvP only and holy was designed for PvE only, and I actually played both at the time. I don't require perfect balance for everything if different specs simply cater to different niches. But then WOTLK came, disc was made to be PvE viable as well and I thought that was great for all the priests who had longed to play it seriously in PvE for a long time... but I have to admit, I'm rather cheesed off that holy never got the same treatment for PvP. Like I said, I'm fine with different specs having different niches, but if you make one talent tree able to do everything while still limiting another, it gets kind of annoying for those who suddenly find themselves left behind.
I know that various damage dealers have had to deal with this issue for a while, but as far as healers go, holy priests are currently the only healers who are not considered PvP viable at all. I don't know why Blizzard can balance all other healing specs around both aspects of the game but not holy priests. It gets really depressing though when you're trying to look up guides for playing rated battlegrounds as a holy priest and find that there don't seem to be any because even the most basic "rated battlegrounds 101" guides assume that you won't want to bring anything other than a disc priest.
Well, screw that, I say! I'll write my own guide then!
As far as credentials go, I'm not amazing at PvP, but I've managed to get Legionnaire as holy this season and I feel that my team's further progress has not really been limited by my spec but that we've simply hit our skill cap. I reckon that a really good player could get much higher as holy still.
Here's the thing: while holy is still not really balanced around PvP, it's actually in a much better position than it has been in a long time. I believe that the bias against the spec mostly comes from two sources: disc priests having greater survivability, and that holy is still more focused on AoE healing, while PvP tends to be more about being able to keep a single target up while it's being nuked. Both are fair points... however there are "but"s attached.
As far as survivability goes, I would still agree that disc priests have more of it, but the difference is not as pronounced as it used to be. When I come up against a team with a disc priest while playing in my (admittedly low ranked) arena team, it never feels to me as if they are that much better at keeping themselves up against damage - the bigger problem tends to be mana regen, as a disc under pressure keeps getting returns from Rapture, which works in a "the more you get attacked, the more you regenerate" way similar to a shaman's Water Shield. As holy on the other hand I'm in serious trouble once I run low, because even with full "of meditation" gear (aka spirit on everything), the rate of regeneration is comparatively slow and if the match continues for long enough, I'll eventually die simply due to being oom.
In rated battlegrounds, with the sheer amount of people involved, survivability is in an entirely different ballpark. A good team can pretty much burst down any healer if they are truly focused. Still, your best bets for survival are paladins and shamans, because they are a bit sturdier due to their armour. Druids and priests of either spec are all considered squishy, with the differences being so marginal as to hardly even matter. I'm still continually surprised by how many teams will go for our resto druid first before coming for me because it just feels weird to not be the primary target.
Also, the holy PvP talent Blessed Resilience works much better when there are other healers around to help you out. Survivability going up when someone else heals you may sound like a bit of a "duh" observation, but compare Blessed Resilience to the disc equivalent Focused Will, which provides the same amount of damage reduction regardless of circumstances. A +30% healing taken buff on the other hand scales in effectiveness the more people you actually have healing you.
As for holy priests being AoE healers... well, that's true, but it's also the reason why the spec works much better in battlegrounds than in arena - there are actually enough people around to make AoE healing worthwhile. You just have to be aware of your strengths and weaknesses and plan accordingly. Leaving your holy priest behind as the only healer to guard the tank against rogues is a bit of a waste, and a paladin or a shaman could do a much better job. As a holy priest you always want to be hanging out with as many people as possible so that your AoE heals get a chance to shine. I love joining the big battle for the waterworks in Battle for Gilneas for example, because Prayer of Mending and Circle of Healing are absolutely perfect for easily healing up incoming AoE damage as the enemies try to prevent you from capping the flag.
Let's look at some holy talents and talk about their best use in PvP.
Okay, this is far enough up the tree that I'm guessing that most disc priests are specced into it as well, but I wanted to mention it because it's a talent that I've always avoided in my PvE specs because I simply didn't need it. In PvP however it's awesome, because it's a sizeable instant heal, and in an era where every single melee class has at least one interrupt on a short cooldown, instant heals are king. I use it all the time whenever I get nuked, which is quite often. I think that it should always be your first choice of survival cooldown, because as opposed to Guardian Spirit it can only be cast on yourself - better to save the latter for someone else if possible.
Now, I have to admit that Blizzard managed something that I had thought impossible with Lightwell this expansion - they actually made it not completely pointless. Still, its actual viability remains highly dependent on how well you can train the rest of your team to use it. I'm quite fond of it in arena, because it's easy to convince one or two other people to use it, and sometimes I simply use all the charges myself to click-heal myself through various stunlocks and silences. In battlegrounds, even rateds with a fixed team, I have to admit that I haven't had as much luck, and even in very heated battles I often see it expire with several charges still unused.
Still, in base-capping games it's worth plopping down a Lightwell near a flag that you're defending or assaulting - only if you intend to stay there yourself though. I learned the hard way that you can't "leave" a lightwell for someone else to heal themselves in your absence as it despawns if you move too far away. In flag-carrying games I tend to put one down near our flag carrier if we're preparing to defend against an incoming assault in the flag room - he's the only one I've seen click on it with any regularity whenever he gets nuked.
Spirit of Redemption
"Improved Death" has never been a favourite of many priests, and I remember intentionally skipping it in my PvE spec for a long time because I didn't want to "reward" people with free heals for letting me die. I have to admit though, in PvP it's quite awesome. In PvE you can argue that counting on your own death is a bad idea since you should always plan to beat each encounter with no deaths, but in battlegrounds in particular, most people will suffer several deaths each game (unless you're dominating your opponent completely) and there's no shame in it either.
In such an environment, Spirit of Redemption is actually a bit of an Obi-Wan Kenobi power: "If you strike me down, I shall become more powerful than you can possibly imagine!" Massive uninterrupted healing for fifteen seconds, hurrah! Considering it's usually quite tricky to get cast-time heals off due to the many interrupts and silences, this is seriously huge. You can generally recognise the smart teams because they'll actually avoid killing me until almost everyone else is dead, so that there's nobody left for me to heal once I'm in spirit form. If I die while there are still people up, I'll spam Flash Heal x2 for Serendipity stacks and then follow it up with a Greater Heal or even Prayer of Healing. If the group is under serious pressure, being dead is a great time to pop a Divine Hymn too.
One thing to keep in mind is that since dying actually provides a (limited) benefit to healing, it goes from being something that just happens to you, to something that you consciously plan for. For example if it looks like multiple people including me are close to going down, I'll stop healing myself and try to die, so I can spam uninterrupted heals on the others and keep them up for longer (which can be quite essential when all you need to do is hold on to a base or keep a flag carrier alive for just a few more seconds).
Resurrect timers are an issue as well. In rated battlegrounds you should always try to communicate them anyway if someone's already dead, so that others can "die up" at the right time and res up again immediately, but this is even more important for holy priests. If another round of resurrections is imminent, it might be better to click off the spirit buff early so you won't get stuck with a 40 second res timer once it runs out. On the other hand if you die and just missed a res, it can be fun to have someone else throw you a combat res, die again, and then spend another fifteen seconds spirit healing while you wait for a "proper" res. Might as well make use of that downtime, no?
Chakra is interesting because the way it was first introduced, it was not very useful in PvP at all. The 30 second duration was simply way too short, and nobody had the leisure to recast Heal or Prayer of Healing to refresh it either. Now that it's got unlimited duration however, you can "buff up" into the correct Chakra state at the start of the match or after resurrecting and then you don't have to worry about it anymore.
When I still ran into other holy priests in our rated matches (haven't seen any in a long time), they seemed to be using the Sanctuary Chakra state most of the time, but I never understood why. Yes, the buff to Renew's output is nice, but the associated Holy Word is quite useless in an environment where you have little to no control over people's positioning and they move around all the time.
Chakra: Serenity on the other hand is awesome, simply because it's an instant heal on a short cooldown, and you remember what I said about instant heals earlier, right? The fact that it refreshes your Renew on the target is just a nice bonus. I pretty much use this on cooldown.
Holy Word: Chastise is worth mentioning in so far as it's nice to have a brief stun/disorient effect if needed, and one that people generally don't expect at that. It's useful to have a macro to quickly cancel your current Chakra state so you can use this as your Holy Word instead if needed; however, it can be a bit of a pain to get back into your desired Chakra state right afterwards once you're in combat.
Circle of Healing
There isn't much to say about this one other than what I already said above about AoE healing in general: try to stick to larger groups so you can get the most bang for your buck when casting this. For PvP it's generally not worth glyphing it for the extra target. Also, if you're really under pressure it's nice to keep in mind that this is basically another instant heal that you can use on yourself, even if the single-target portion of it is relatively small.
Once again, similar to what I said about Blessed Resilience above, this is not as powerful as Pain Suppression in PvP, because preventing damage is more powerful out of the box than increased healing. Still, it's a pretty decent survival cooldown to use on a beleaguered flag carrier... or anyone else for that matter. I have it glyphed for the reduced cooldown because unless I'm anticipating an attack on our flag carrier, I'm pretty much using it on cooldown. Glyph of Desperation is a must-have as well - nothing like seeing someone be sure of their kill as they apply a stun on you on low health - and suddenly the wings go up and you're saved.
In general however I actually avoid using Guardian Spirit on myself, because I have Desperate Prayer as a survival cooldown for that purpose, and because as I mentioned in the Spirit of Redemption section, for a holy priest in PvP dying can actually sometimes be a good thing. (And you can still cast Guardian Spirit on someone else while dead/in spirit form!)
Anyway, this is just an attempt to summarise my own knowledge about holy priesting in PvP, as I've acquired it over time. There needs to be more information about this out there! I'm open to feedback or links to more resources if anyone's got them.