By taking advantage of brand-new hardware separate from your realm's instance servers, not only will you be able to find groups from pools of players in your entire battlegroup, your realm's specific instance servers will be free of the stress of mass heroic-running.
On top of the convenience factor, Blizzard says that rewards will be in place for players who lead groups through instances successfully. The nature of the rewards is currently unknown but Blizzard promises more information soon.
Blizzard says that the feature will be used for 5-mans only at this time. It'll be launched with 3.3, not with Cataclysm.
Fantastic news! No more "additional instances" errors, a larger pool of players to choose your groups from, and rewards for good party leaders. Great news for everyone.
Source: wow.com
