Vanilla, the whole world mattered. Then, afterward... Hellfire Peninsula & The Throne of Kil'Jaeden, Shadowmoon Valley just outside of the Black Temple Entrance, the Isle of Quel'Danas, the Throne of the Elements, Ring of Blood; all highly popular pvp areas aside from the other content they offered (world bosses, daily quests, etc.). Wintergrasp, the Argent Tournament, Zul'Drak Pa-Troll areas, Argent Vanguard grounds at the foot of Icecrown and the Ampitheater of Anguish; Hyjal beneath Nordrassil, the Molten Front, Thundermar and Bloodgulch in the Twilight Highlands, Ramkahen and the river delta in Uldum, Tol Barad, Therazane's Throne; the low lands of Kun'Lai Summit, Peak of Serenity, Tavern in the Mists, the entirety of the Jade Forest including Dawn's Blossom, Windward Isle, and the Arboretum; Halfhill, the low lands of Krasarang Wilds including Lion's Landing and Domination Point; Zan'Vess, Klaxxi'Vess, Lake of Stars, Terrace of Gurthan, Niuzao Temple, the entirety of the Vale of Eternal Blossoms, the Isle of Giants, the Isle of Thunder.
That's up to MoP, things that just came to mind, and doesn't even include major cities. All of which are open world areas that are popular for one reason or another. So, um, yeah, nice troll attempt.