They are changing how raid bosses play out drastically. Every spec is getting a new ability that triggers when certain conditions are met. Managed to find out couple of them.
Demonology Warlock: Call Pit Lord
Taunts and deals massive dmg to the target for 20 seconds. (This works against raid bosses too)
Shadow Priest: Pareidolia
Creates void area in the ground. While standing on it all spells cost health instead of mana, but you are immune to all other damage for 10 seconds. Benefits allies too.
Restoration Druid: Tornado
Pulls everything inside and makes them airborne. Increasing dmg they take from spells by 30%.
Outlaw Rogue: Pillage
Calls a ghastly pirate crew to harass all enemies. They randomly gouge, cheap shot and blind enemies. CCd targets get double benefit from prey on the weak.
Frost Death Knight: Vile Spirits
Summons Vile Spirits that charge towards enemy and deal massive aoe dmg.