Priest have been nerfed. They were quite strong before: high damage, good heals, good survavility, not very mana efficient but had mana burn. Now, nearly every dps class can one-shot you, heals are strong but if you're stunned, silenced or interrupted more than 5 sec, you're probably dead. Mana efficieny is quite bad and mana burn is gone.
I've always preferred discipline and penance is a great ofensive ability (not that good defensive right now, flash heals heal more and faster, anyway it's a good fakecasting tool), but I'm playing as holy because it's the only way I've found to survive a lil more. Holy playstyle is very defensive, dps is low but heals are great. You get an instant heal, nice for dangerous situations, but what surprised me the most was lightwell. It's great, heals a lot (about 500 per tick each 2 sec) and you can click it when you want: while silenced, while stunned, even while casting something else. Serendipity is quite good too, casting greater heals in 2sec. Also you get that awesome 3sec stun that can be used in so many situations.
I just found that as discipline I lacked of enough survavility to do anything, so I just changed to holy. Anyway, you can spec whatever you want, today a feral druid critted me for 3'5k damage... (lol, I only have 3k self-buffed).
Anyway, priest are not that bad, they've strong heals, dispells and can help a lil with CC and damage, but survavility is their major weakness.