Hmm, off the top of my head, I'd deal with this by creating quests that are cyclic. Let's go back to the "defend a village" example in the article. Say Player A gets a quest from the village head to defend a village. He kills off 5 bandits attacking the village and the rest of the bandits run off. This can trigger off a number of potential quests for Player B, who's on the side of the bandits. He can get quests to (for example) gather supplies for the bandits, recruit more bandits, kill Player A, and eventually lead another attack against the village. And this triggers off the initial "defend a village" quest again. So no completed quest objective is truly permanent.