Lord of the Rings Online. The system they have in place now is very unlike what most other games consider a player vs player environment. To many, this is a good thing, but to others, it detracts from the game. To fully understand the reasoning behind no open PvP system in LotRO, one must first fully comprehend the lore. J.R.R. Tolkien's world is extremely important to Turbine (despite what some may think with the introduction of the Rune-keeper), so the way they incorporate popular game mechanics into their heavily lore-based PvE game is a constant battle.
Over at Pumping Irony, Scott gives us his impressions of how this could all work. He talks about all of the reported changes coming to PvMP with Mines of Moria, and mentions his understanding of the lore and how the current world of Eriador is mostly unaware of their impending doom from the forces of Sauron. This is an excellent point as many players have no idea that this is the case. Scott also brings up some great points about the future of LotRO PvMP and how Jeffrey Steefel himself has hinted at an expanded system coming after Moria. Check out Scott's post and let us (and him) know what you think.