I think 85 is awful. I'm rolling another disc/shadow priest to play at 70s xp-off.
Healing heroics (and even regulars to some extent) is a painful helpless feeling, and the 85 BGs are FILLED FILLED FILLED with bots, afks, and "just let them win" griefers who are trying to get their honor points and don't want to play the game at all.
I sometimes enjoy the level 85 dungeons on my pally tank, but I'll never heal a Cata LFG again in any class/spec. Arena on my pally is doable, but that's not really my gig, and I don't enjoy it at all as a disc or shadow priest.
If I could demote this toon to 70, I would. I'm really hoping that's the promised land everyone says it is.
I don't know much about 70s yet, but my advice would be to park there until you're really sure about going to 85. My 85 priest is quickly becoming my bank alt.
There is a 70 twink thread in the BG forum. It's rather robust and has a lot of silly dialogue, but if you genuinely ask for tips on twinking, people will readily help out. We want more twinks, so when people indicate they are considering it, the community responds.
The thing about 70 PvP is, because of wrath gems and enchants (primarily resilience gems), you significantly reduce incoming damage. So altho a lot of twinks like myself can burst for incredible numbers, combined efforts and smart playing make it a real struggle to bring individuals down. The battles are essentialy epic between equally geared individuals, and it is obvious who is not a real twink and who is, as the long time twinks like myself primarily work together in the BG's.
In fact, if there is any true griefing in 70 Twink BG's, it's towards the wanna-be's who treat !xp BG's like leveling BG's. You simply have to adopt a more team oriented mindset. There is a profound lack of afkers and bots as well.
Since anyone can essentially purchase a full set of the best lvl 70 pvp gear once they hit 70, it's not about gear beating people so much as it is about coordination and ability. That's why I enjoy it so much. It's the interrupts and dispels and coordinated spikes that lead to success, and that takes teamwork and good awareness.
Until you are geared tho, it can be very unenjoyable for you, as you will often be 2 shot. My advice to anyone intending to do 70 twinking is, BG from 60-70 and accumulate all the honor needed to purchase a complete set of 70 PvP gear. Plan it ahead of time and have your gems and enchants ready as well.
Don't show up at 70 having no honor, turn off xp, expect to do twink bg's with pve gear. Even prot paladins and prot warriors and blood tanks get 2 shot. You'll be massacred if you do that. So come prepared. It's actually so easy to kill people like that for individuals like myslef, we kinda feel sorry for you.