Blizzard has been promising changes to raid buffs and raid stacking for a little while. Last night, they laid all their cards on the table in a giant blue post. Here's a run-down; all of this will be going live in 3.0.2, the pre-Wrath of the Lich King patch.
- Almost all buffs and debuffs will work raid-wide -- Warriors shouts, Shaman totems, Paladin auras, and more.
- There are approximately 30 categories into which raid buffs and debuffs fit. All raid buffs/debuffs have been put into one or more of these categories. Most categories have only one or two buffs in them; see the post for the category breakdown.
- Buffs cannot stack within each category; only the most powerful one will apply. For instance, Battle Shout and Blessing of Might will no longer stat.
- "Mana battery" action (such as Vampiric Touch) is changing significantly. It won't depend on damage done any more. Whenever such effects occur, the ten people in the raid with the lowest mana will receive a buff called Replenishment restoring 0.5% of their maximum mana each second. This buff will be provided by Shadow Priests, Survival Hunters, and Retribution Paladins.
- Heroism/Bloodlust will now affect the entire raid, but you can only be hit by it once every five minutes.