Mythic's Dark Age of Camelot always depended upon its PvP to set it apart from other games. Not just killing random players from opposing factions – and DAoC had three – but actual, full-out siege warfare, battling for possession of inter-realm keeps and artifacts of great power.
The nations would form outside a wall called Emain Mecha, within sight of the Hibernian realm, close to teleporters from Midgard and Albion. And there battle would rage for hours, with people of all levels and classes fighting openly or sneaking behind for snipe shots from hiding; often times victorious Albions or Midgards would break through the Hibernian line and assault one of the nearby contested keeps. Other times, the Hibernians would take the fight back through the teleporters to the other realms.
DAoC also had the battlegrounds, which they expanded over time to provide challenges for all levels. You could fight against the NPC critters there, but glory came in the taking and holding of the central keep. Unlike the frontier keeps, the center keep was meant to be easier to take, and control could shift many times. It was a battle which never truly ended, but sometimes a realm could hold it for a few days. WoW's battlegrounds? DAoC had them years before – and arguably better.
They also introduced a contested dungeon – Darkness Falls, den of much treasure and experience. The mobs (and enemy players) there also dropped signets that could be used to buy even nicer items. The entrance shifted among realms depending on who controlled the most keeps in the Frontier. If you wanted to get this good loot – you helped your realm hold keeps. DAoC had decent PvE content and a number of expansions, but they always came back to their core, their PvP. Why wait for Warhammer Online? You can get in the battle right now. If you play Warcraft for the PvP, you owe it to yourself to see PvP done right.