While there's nothing better than popping that Icon of the Silver Crescent and dotting a mounted opponent up in Alterac Valley with Devouring Plague, Shadow Word: Pain and Vampiric touch to get that Sickly Gazelle Achievement (In Alterac Valley, kill an enemy in the Field of Strife before they dismount), for the undergeared (i.e. low resilience/hp a la PvE gear), Disc PvP healing can also serve up a decent helping of fun.
To give a frame of reference, I was running around in various Battlegrounds this weekend using a 51/10/0 spec on my ~1000 Spellpower, 178 (or sometimes less) resilience, 8k hp/mana baby raid face melter. Instead of melting faces, we were (literally) channeling the Light into our teammates and trying desperately to avoid getting gibbed. In some cases, we were forcibly shoving the Light into opponents with some pretty righteous fire. All in good fun, of course.
With the above spec, we primarily relied on the tried and true instant cast abilities to save lives: Prayer of Mending, Renew, and Power Word: Shield. However, a couple of new talents completely changed the old Disc PvP playstyle. The most amazing spell in our new repertoire is the 51 point Discipline talent Penance (16% base mana, 2 second channel, 30 yd range): Launches a volley of holy light at the target, causing 288 Holy damage to an enemy, or 1484 to 1676 healing to an ally every 1 sec for 2 sec. Now, at first glance, it doesn't sound so amazing. Drawbacks? Yep, right here: a) It's channeled so you have to stand in place, b) being channeled you could possibly get counterspelled and school locked, c) channeled means you have to face the target and d) there's a cooldown of 8 seconds with 2/2 Aspiration (Rank 2) Reduces the cooldown of your Inner Focus, Power Infusion, Pain Suppression and Penance spells by 20% which really is 6 seconds if Penance is fully cast.
But, enough about drawbacks, lets talk about the advantages of using Penance:
Firstly, it can be used as an offensive weapon. Need to apply some more damage to finish off the opponent? Penance them! Every other aspect below also applies to the offensive usage of this spell.
Secondly, Penance is cheap. For a mere ~460 or so mana, at 1000 spellpower, each of the 3 ticks heals for approximately 1500. That's around 4500 healing for less than cost of a Flash Heal (18% mana). This means that if you have to break the channeling after the second tick of Penance, you will still heal as much as a Flash Heal, for less mana, and for less time spent casting (first tick is at time zero, second tick is at 1 second).
Thirdly, the initial tick of Penance is instant. The moment you start channeling, that first tick shoots out and while the range is only 30 yards (most other heals have a max range of 40 yards), travel time isn't so bad. It's not as slow as Pyroblast and the twirly lights are pretty. With latency, its entirely possible to get off two ticks, break the channeling, and still avoid counterspell. Basically, it looks like a holy version of Arcane Barrage, which if you've participated in BGs lately, have probably been subjected to, ad nauseum.
Fourthly, all ticks can crit. This means that with some luck, you're looking at some pretty amazing burst healing.
Fifthly, it synergizes well with Grace (Rank 2): Your Flash Heal, Greater Heal, and Penance spells have a 100% chance to bless the target with Grace, reducing damage done to the target by 1% and increases all healing received from the Priest by 2%. This effect will stack up to 3 times. Effect lasts 8 sec. The 3 ticks of Penance count as 3 heals, applying (very quickly) a 3% damage reduction and a bonus 6% healing.
Lastly, being a channeled spell, you can heal around objects if the subject of your healing moves out of your line of sight. This is akin to a Warlock's Drain Life, which, if you start the channeled spell when the target is in line of sight and they move out of your line of sight, the channeling will continue to function. This is very helpful as in BG's, there's a good chance that your teammates won't be mindful of line of sight healing issues. In addition, if you're are playing with a teammate that is communicating well with you, they can pop in and out of line of sight, so that you can protect yourself from counterspell while still casting large heals.
Ok, so Penance is great, what else? Well, check out the new 46 point talent: Borrowed Time (Rank 5, 5 point talent): Grants 25% spell haste for your next spell after casting Power Word: Shield, and increases the amount absorbed by your Power Word: Shield equal to 40% of your spell power. While the second part is easily quantified (1000 spellpower = 400 extra damage absorbed) and works well with Reflective Shield, the first part (spell haste) can bring about some pretty fun stuff. With this 25% spell haste and no other passive haste or other haste buffs, Flash Heal becomes a 1.125 second heal, Talented Greater Heal becomes a 1.875 second cast and Mind Control (*cackle*) becomes a 2.25 sec means of humiliation. The cool thing about the haste ability is that it can stack with Power Infusion. This next spell becomes 45% faster, or as they would say on Spaceballs: "They've gone to plaid!" Mind Control. check. 1.65 second cast. check. The important thing to realize with the Borrowed Time haste effect is that any spell, even instant, will consume the buff. So, when planning out healing rotations or combinations, be sure to avoid casting any instants right after a shield if you want the benefit of the haste effect.
As nice as the haste effect is, Borrowed Time and Penance both synergize well with yet another talent: Rapture (Rank 4): Causes you to gain up to 2% of your maximum mana each time you heal with Greater Heal, Flash Heal or Penance, or damage is absorbed by your Power Word: Shield or Divine Aegis. Increasing the amount healed or absorbed increases the mana gained. Any form of mana refund to bolster the efficiency of the Priest class is a phenomenal thing and this talent is a very nice complement to the efficiency of the above talents.
I definitely had a lot of fun with this spec and I urge people to give a whirl. Healing becomes quite effective with all of the new talents in the Discipline tree and with 5/5 Divine Fury, if you want to go offensive you can, with a Penance, Holy Fire, Mind Blast, Smite chain for some Light guided beat down.