I'm looking at the new trees and am honestly excited by what I've seen and heard. Note this is NOT by what is currently on the calculators, in order to see the exciting bits you have to acknowledge what is coming and also what will likely come. I'm reading between the lines a lot for this, but anything I say here I deem likely to happen via experience from the other expansion releases plus what Blues have been saying.
The return of Atonement: Blues have already stated more than once that Atonement will be a glyph, not a talent, because it would not be useful to all three specs. Whether they need to keep this line of reasoning with the other talents is immaterial, because it means that Disc (and possibly Chastise Chakra Holy) are going to be able to go Atonement plus Vampiric Dominance and splash 145% of the holy damage they deal entirely as smart heals, and 45% of everything else. The question is whether these heals count as "direct" for the purposes of Echo of Light, if so then Holy will be concentrated insanity (maybe even going to Serenity Chakra to keep renews rolling on a TON of targets)
Atonement healing was my favorite part of being a priest, one that sadly couldn't keep up once I crossed level 80. The opportunity to do so in raids would make my Disc Priest my main forever more.
Shadow: There is no way we've seen everything in Shadow, it isn't possible. We specifically haven't seen the core of the Shadow Orb generation mechanic. According to Blizz, they're attempting to take some of the RNG out without removing it all. I can think of about four ways to achieve this off the top of my head, I'm sure Blizz has a way or two by now. The spec loses some in the utility department (though not much, to be honest, we still have Flash Heal, dispel/Mass Dispel, and PW:S plus anything from talents). We also need to see how Evangelism works out. I'm also hoping the Shadow Orbs turn into a resource bar, ensuring that they won't be able to be dispelled.
I'll admit to not being too solid on shadow-side PvP (I'm Holy for that), but I see almost no major losses for PvP purposes (especially if the Psychic Fiend isn't killable). If anything, there are a few new MAJOR tricks in the the talent tiers for anyone wishing to save an ally while in Shadow (Void Shift springs forcibly to mind). The only concern is the loss of Devouring Plague, but depending on how Vampiric Touch shapes up that will be a moot point. Dispel changes are not a major detriment to the Shadow Priest, either, just a playstyle change more towards dispersed DoTs (Shadowcleaves) which will be near-impossible for a single healer to cleanse and will cost valuable cooldowns and GCDs in in 5v5.
There is also the fact that there are at least 5 major playstyles available among three specs, probably much more than that once we get into the nitty-gritty. This is a common complaint among several other classes, that their specs are forcing them out of beloved variations. I don't see that here, every variation of Priest I've played in is available plus extra perks via this system.