Spent all evening bashing out my Disc priest PVP spec, gear, and interface, and then testing him out on a few battlegrounds and Wintergrasp. Thoughts:
1) Grid is borked of course, so I reverted to the (improved) native Raid UI and... liked it! It isn't as flexible as Grid but I was able to get everything out of it I needed, with a little tweaking. I may stick with it for a while just to avoid patch-day madness of losing my most critical interface element for healing anything bigger than a five-man.
2) After being spooked by all the reports that healers had mana trouble I picked all of the Disc talents that reduce mana costs or return mana to the pool (Mental Agility, Archangel, Rapture, Power Infusion). I'm running in PVP Mooncloth gear, my total Spirit is 690 and Intellect is ~1980 unbuffed. I had NO ISSUES whatsoever with mana using more-or-less my regular casting schedule, i.e. heavy emphasis on shields, Penance, Prayer of Mending, Renew, and Flash Heal, mixed with dispels and offensive casting. I had one notable Warsong run in which myself and about five teammates held the horde side of the field for six or seven minutes. I cast perpetually and never dipped below about 70% of total mana. At the moment I would say I CAN'T run out of mana in PvP unless I was subject to one or two very determined opponents spamming mana drain. I noted the same on the other side, at least regarding well-geared priests: They were rarely low on mana, and even mana drain made little impact.
3) I'm more fragile in PVP, following changes to offensive abilities all around and to resilience specifically. I was very hard for rogues to kill one-on-one prior to the patch: I got properly ganked a couple times last night. I think my lifespan will undoubtedly be shorter. Whether this is a "problem" rather than simply an adjustment remains to be seen. Prior to the patch, a friend and I have been running guerrilla campaigns in certain battlegrounds (EoS, AB). His Prot Pally and my Disc Priest would charge into enemy control points and simply dare the enemy to kill us, and it typically took five or six of them to overwhelm the two of us: we could hold out for ages against an equal or slightly larger force. That's not going to be possible now, Disc can no longer "tank" PvP, or at least not to the same extent. I'm sad to see it go. On the other hand I'm sure all the hunters, rogues, death knights, and warriors who couldn't scratch our Prot-Disc tag-team are rejoicing.
4) I'm struggling to get used to some of the new talents and spells. So far, I love the idea of Archangel, and when I pull it off for 5 stacks of Evangelism I feel like a miniature deity. It's a fantastic extended reward for flipping to the offensive. For whatever reason I'm pumping out considerably more damage, too, without really trying. I failed to use Inner Focus successfully, though, and therefore never saw much advantage to Train of Thought. I always have to condition myself to actually use abilities on medium-term cooldowns; I plan on working on that tonight. At the moment, though, I don't feel that I need the extra efficiency. I tried casting Heal now and then, felt very very slow in a PvP environment, though that's in part due to my limited Haste.
5) On the topic of absorption spells and the Mastery stat. Disc Priest absorption spells become stronger with increases in the Mastery stat. I reforged all of my equipment to transfer a portion of Haste to Mastery, driving the stat up from the base 8.0 to about 11.5. This represents a total increase of 29% to Power Word Shield, Divine Aegis, and Power Word Barrier. I definitely noticed the difference in battlegrounds and I'm now trying to decide which other stat I can sacrifice to get Mastery higher... up to 18 or so, ideally. Given the mana situation I may reforge again and exchange Spirit for Mastery, reverting the Haste.
6) Finally, I looove Power Word Barrier. I've been skeptical whether the spell as designed was worth spending the talent point. After last night, I'm a believer. It's large enough that it can temporarily change the course of a battle, which feels right for a signature spell on a three-minute cooldown. Beyond the stats, it has a useful psychological impact on other players. I can use it to rally friendly players to an important skirmish, since people tend to run into the zone. It prompts enemies to burn crowd control (fear in particular) to attempt to drive people out of it. At the moment some ranged DPS aren't even trying to bash down the barrier and will concentrate instead on players standing outside it, giving healers ample opportunity to top people off. It's great.