So I decided to hop on my level 21 priest tonight and try out the new systems for PvP...mother of god. Disc is fucking godlike. The entire WSG match, I didn't die once, had infinite mana, and got #3 on damage and #1 on healing. I wasn't even twinked either, I was just wearing account-bound staff/chest/shoulder/helm/1 trinket, the rest were greens and even a couple whites.
At one point, I was tanking two rogues and a hunter on me, just spamming myself with Flash Heal, due to there being no interrupts. :P
Also, Psyfiend is another incredibly OP thing. Plop that thing down and completely screw an entire group of enemies. Is this what pre-3.0.8 Divine Hymn was like?
Biggest improvement over pre-MoP I'd say is the mana. All mana costs got reduced it seems, because I had infinite mana. Literally, I never even came close to running out of mana, because I regen my casts within seconds. Whereas in Cata, I'd cast like 10 spells and be OOM, and have like 15 mp5 as my regen...just running around waiting for enough mana to cast again.
I'm surprised they didn't nerf Penance's damage after nerfing Holy Shock and such. Penance still does around 60-70% of an equal level enemy's HP. Smite and HF hit decently, but SW: P ticks for 40 or less.
Just wanted to gush a bit, because I've never felt this OP in a battleground ever since the middle of patch 3.3. If you've got a low level Priest, hop on it and go play Jesus in WSG!