World of Warcraft has single-handedly made me hate the idea of factions in MMORPGS.
I was reading this post on Corgi Island about the idea of cross-faction raiding in WoW, using RealID. (Fun new WoW blog, BTW, if you're interested.) I posted a comment which I am re-posting here with some elaboration.
Gloria mentioned in her post how silly it is that she can't muster all of her Real Life friends together to run a raid, especially given declining populations in WoW, added to the fact that the "War" in Warcraft has been largely absent for the past couple expansions anyway. World PvP is dead, partly because of the population and partly because of changes made by the development team. *cough* Southshore/Hillsbrad *cough*
I've heard The Secret World has (will have) inter-faction PvE. With three factions, supposedly PvP can be more easily balanced because the weaker sides and always gang up on the strongest sides. (Can we call that the DAoC Doctrine?) But then, when the shit hits the fan in the PvE story (Lovecraftian horrors consuming the world?), players of different factions can put aside their differences and work together for the greater good. This is something I've always felt was missing in WoW; and it was worse in Rift, where I always felt the two factions had no business fighting each other when their world was facing an existential threat.