Wowstiq has posted preview of the T12 Set Bonuses. A lot of them are nice, but two in particular stood out for me.
The first was the Shadow Priest 4pc bonus:
4-piece While you have Shadow Word: Pain, Devouring Plague, and Vampiric Touch active on the same target you gain Dark Flames, which reduces the cooldown of Mind Blast by 3 sec.This is a really interesting direction that I don't think I've previously seen in set bonuses. Most set bonuses give you some kind of passive bonus for doing the things you already do (i.e. Resto Shaman 2pc: Your periodic healing from Riptide has a 40% chance to restore 1% of your base mana each time it heals a target) or significantly alter your standard rotations (i.e. Shadow Priest T9 2pc...that extra 6 seconds really screwed me up for a while. I had the first minute of my rotation ingrained into muscle memory.)
While this new Shadow 4pc bonus works similarly by reducing the cooldown on one of the most powerful Shadow Priest spells by a significant amount, it takes it one step further. You have to be playing your class right to get that bonus. On a boss fight, a Shadow Priest should have near-100% uptime on SW:P, DP, and VT. This set bonus further encourages people to learn how to do that in order to make the best use of it. It's a really interesting direction for Blizz to take with the design and I'm very interested in seeing how well it works out.
Similar to that is the new Cat 4pc Bonus:
Your finishing moves have a 20% chance per combo point to extend the duration of Berserk by 2 + (20 * ComboPoints) sec...While I'm pretty sure the number 20 is a place holder for something much more reasonable (not that I'd complain about a 102 second extension to Berserk) it follows that same trend of trying to get us to play our class the right way.
Common wisdom right now is to Shred like a manic while under the effects of Berserk, ignoring finishers (except to refresh Rip if it's about to expire). While it's certainly a huge DPS spike, it pretty much ignores the way the combo point system is supposed to work. This set bonus encourages Kitties to spend those CPs on finishers (probably Ferocious Bites) in order to try to extend the overall duration of Berserk.
To make this work, though, Blizz needs to really fine tune it. FB isn't in the best of places right now. There are some people who suggest it should be ignored outright until the last 25% of a fight (given 2/2 Blood in the Water). If it's not glyphed it's a very high-cost attack. And if it is, its overall damage potential isn't much better than Shred.
Even under the effects of Berserk, when the Energy cost is reduced, there still needs to be a viable trade-off.
We know that whatever the final number on the time extension ends up being, they're not going to make it high enough to guarantee another 5pt finisher before Berserk expires. Otherwise we could keep ourselves in a Berserk state for the entirety of a fight. But it still has to be sufficiently long for using FB when you already have Rip and Savage Roar going to be a good exchange for those extra seconds.
Or they could just retune FB to make it more viable before the last 25% of a fight...
Complete tangent aside there, it still shows an effort to get people to play Kitty DPS the way it was intended: build your CPs and spend them.