I think that there should be a PvP tag for World of Warcraft, wow-pvp or world-of-warcraft-pvp.
There are currently 77 questions regarding PvP in WoW, showing that there is a lot of interest. In-game, PvP and PvE require differing approaches, meaning that a question warrants a different answer in a PvP context than out of it. Also, I personally have wanted to use such a tag but, seeing that there was only a world-of-warcraft tag, I was forced to clarify my interest in a PvP-related answer in the question title.