Recently the new essence received from PvP which gives you access to an extra PvP talent, that also works in PvE situations, has made waves and there are plenty of threads about it on the forums. One in particular got a blue response which clarified just how powerful and how relevant the essence is supposed to be for PvE-focused players. Each spec gets a specific PvP talent added and some of them currently selected on the PTR are a little too powerful and feel mandatory for PvE, such as Moonkin Aura for Boomkins and Leader of the Pack for Feral Druids, but Blizzard clarified it is not their intention to have must-have talents for PvE available through the essence, as its primary goal is to add additional choice for PvP players.
Blizz stated in a watercooler that they're using the 1.12 database as a starting point. I don't think it's a stretch for people to assume that most game features and mechanics will be in their 1.12 state. The kind of stuff I would expect to see reverted would be numerous AV changes, but I seriously doubt class mechanics are going to be reverted. I'd be surprised if the threat changes that ruin pre-naxx encounters are reverted.