If you ever played during The Burning Crusade, you likely remember the days of world PvP objectives. Whether it was the back-and-forth of Halaa, the odd scout-chasing game of Zangarmarsh, nabbing bases in Hellfire Peninsula or simply capturing towers in Terokkar Forest, players indulged in PvP at their whim, and it had an effect on the world around them as they played. This interesting, interactive approach to world PvP was generally well-received, if a little frustrating at times.
Wrath of the Lich King moved away from world objectives in favor of world PvP zones by introducing Wintergrasp. This outdoor battleground was a source of not only PvP rewards and mobs to farm, it also opened up a raid instance to whoever had control of the battleground at the time. It was continued in Cataclysm with Tol Barad, but Mists didn't appear to have any world PvP element at all.
Until, that is, the upcoming patch 5.1.