For me, Season 5 was all about figuring out how to play the Priest and getting the macros/binds down as it was my first season piloting the class. Theoretically, I understood the class and the gameplay, but my execution of the class was beyond terrible. As I've been playing a Druid for the better part of a year and a half, I tried to play the Priest like a Druid and constantly found myself in hot water. The key misunderstanding was that I thought Priests needed to position defensively (aka Druid-esque) as opposed to positioning offensively. As such, I don't think I was able to maintain a 1700 rating as either DK/Priest or Warlock/Priest.
Season 6 gave Priests a little bit of a bump, but more than the increased efficiency to PW: Shield and a lower cooldown on a now self castable Penance, the biggest change to Priest viability was the overall lowering of burst damage and the nerfing of Paladins' mobile healing options. With those two big changes, Priests weren't getting gibbed in seconds any more while their partners would now be able to pressure opposing DPS.
With this extra time to live, average Priests have recently had the option to play offensive with Dispels, Mana Burns and added DPS, instead of frantically spamming heals in survival mode. This has resulted in Priest representation skyrocketing past their numbers in Season 5. I use the term 'average Priest' because top Priests with top teammates could always make time in Season 5 to play offensive while average Priests struggled to stay alive. Top coordination and gear can create openings for that Burn or Holy Fire.
Me? I'm an average Priest and the Season 6 changes have helped me in an overwhelming manner to overcome the feeling of 'OMG I'm gonna die!' and have given me options in my gameplay. My Ret Paladin buddy and I started our team at zero rating and for the first week, ground up to around 1500. In retrospect, this was a huge error in judgement as it only allotted us ~200 Arena Points for the first week. Even though we hit 1900 in the second week of play, the points gained didn't allow us to buy our Tier 1 Furious weapons until week 3. We'll have to remember this for Season 7. By week 4, we had hit 2k and did it in less than 150 games.
Overall, I believe Ret/Priest is quite a powerful composition. Double defensive dispels, chainable CC and ridiculous burst offers a pretty workable solution to many of the other comps on the way to 2k, but it wasn't until ~1900 that we had to play fast. Playing fast meant that it was necessary for my Pally buddy to Hand of Freedom/Dispel me instantly out of Kidney Shot or Hammer of Justice and necessary for me to instantly Dispel Fears, Sheeps, Divine Shield, Innervate, etc. Prior to 1900, we could play sloppily and the comp strengths would help us out of the various situations.
The need to play fast is due to the fact that teams at around 1900 start to play quickly as well. Opponents at this rating level generally know what they're doing and are often using every GCD to their advantage, trying to push for a little extra momentum that can force a defensive cooldown that can eventually lead to a kill. By playing quickly, you can either force the momentum in your favor, remove random factors, or force the opponents to waste their offensive cooldowns.
Here's a few examples: There were a few times where I didn't dispel the opposing Priest's Shadow Protection fast enough and that resulted in a random failed Psychic Scream and a loss. One or two Holy Paladins swapped auras to Shadow Protection Aura as I ran towards them, which led to resisted Fears. Warriors know how to intervene and reset fights and Druids start learning how to pre-hot, kite and Bear Form to avoid burns.
Throughout the grind to 2k, I started wondering how we could speed up our gameplay and one of the revelations I've come up with is this: DPS often expects healers to perform instantly: instant dispels, instant buffs, instant CC's, making sure Tremor Totem is down, Shock/Ground/Shock combos, Cyclones, prehots, etc. Often, the expectation is that the DPS shouldn't have to ask for these types of supporting actions; the healers should automatically be performing these actions. However, we healers should expect the same kind of support from our DPS. If I didn't have to ask for HoFreedom, Dispels, Intervenes, Fears, focus Pummels/Kicks/Mind Freezes, etc. our gameplay would be much, much faster.
Granted, certain defensive cooldowns should be coordinated, but is it unreasonable to ask our DPS to have the same level of global awareness as us?
I don't think so.