Most people in this thread are mistaken. Mainly because most of their gaming experience only comes from World of Warcraft. You are concerned because you saw how Warcraft Arena play panned out.
Balancing the game around PvP is absolutely fine. Balancing the game around 1v1 , 2v2, and 3v3 small scale situations is where you run into trouble.
Games do not have to keep up the constant balancing act if they balance PvP around group play. This means balancing overall class mechanics and how they play in a full group for PvP, not how they play against every single other class. I would assume this is going to be the route that will be taken considering DAOC and WAR did this and are Mythic games.