World PvP seems more much interesting on Rift. While WoW originally had a few world bosses, modern WoW raiding is entirely instanced. In rift, the rift events are all outdoors and offer very significant rewards in both XP and items. This means that they could become very contested events with world PvP, with a real reward for the winners.
That said, I am only level 29 (50 is cap), so I am not sure how things will turn out in the endgame.