A few reasons we hope it won't be imbalanced:
RvR (Mythic's version of PvP) has been designed into this game from day one, unlike WoW where PvP was an afterthought. Balance REIGNS around PvP here. Don't like it? Go play WoW! That makes this game a little more hardcore as far as balances and such, and the team is aware of it.
Dogs of War. They've been veryyyyyyyyyyy silent on this recently, so hopefully they work out (although from the beginning I've been skeptical). These NPC's would be added to a party in RvR Battlegrounds, adding computer AI and firepower to the team with less players. They've stressed that there would be no way to control these dogs, and that scares me. Tactics reign supreme in WAR, and we don't need a dumb AI breaking team tactics because it can't figure out what we're doing! We'll just have to wait and see.
Finally, DAoC. While people can spout numbers and complain about imbalance the fact remains that DAoC is the only completely game with PvP around where each realm has access to different classes, equipment and spell sets. This is NEVER going to be 100% balanced, players just have to deal with it. I have no problem playing DAoC in any realm, and doing well in PvP....so it can't be that bad!!
Good question though! :)