Before the introduction of the Dungeon Finder in World of Warcraft, players who wanted to form parties for dungeons were limited to a pool of players on their server. This usually took a while to form parties and could sometimes result in improperly geared players or players who did not know what they were doing and wasted everyones time. Looking to expand the Dungeon Finder's cross-realm functionality, it seems that when the next WoW expansion, Mists of Panderia, has been released, that functionality will be introduced to low-population areas whereby players from different realms can group together to complete quests.
Ubergizmo, Cross-realm zones coming to World of Warcraft: Mists of Panderia beta