From my point of view, RvR just won't cut it. I've seen statistics on PvE/PvP, and the majority of players lean towards PvE more than PvP (often just casual PvP, if at all).

A game will have to offer a whole lot more than RvR to stand a chance of stealing some of WoWs customers, especially considering Wrath is out a few months after Warhammer launches.