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.

Many existing buffs and debuffs have been reworked for this new system, in addition to being fit into categories. Here are the big changes to existing abilities; for the full list, see the blue post.

• Shadow Weaving and Improved Shadow Bolt will now only benefit their casters. This is also the case for Expose Weakness and Shadow Embrace.
• Improved Scorch now increases spell crit chance against the target.
• Rampage now increases melee and range crit by 5%, for the entire raid.
• Tree of Life aura now gives every raid member 3% healing received.

Zarhym emphasizes that performance will be thoroughly evaluated for all classes, and tweaks will be made as necessary to maintain balance. The purpose of all this, of course, is to make raid composition less vital for min-maxing and to let you bring whichever players you want (within reason). I think this is a great move, and am really looking forward to seeing it in action.