RPG - Quests that Affect the Game Path
Gamespot has an article about how Guild Wars 2 will utilize quests that have a real effect on the game;
"GW2 will be populated by numerous dynamic quests--quests that are ongoing for the player to discover. The example given was of a pack of savage centaurs terrorizing the countryside. Players will battle these beasts for control of a settlement. Anyone can jump in once the fighting starts, and the event will scale according to how many "active players" are present.
If the players lose the battle with the centaurs, then they lose the use of that settlement. This marks the beginning of a quest chain and advances the scenario to the next level. Now, the centaurs have a foothold and will attempt to reinforce it by collecting wood from the forest. The players can engage them again in the woods, and whether they win or lose the chain will advance in another direction. This constant splitting of the chain means that once the chain resets, the content that can be experienced could be completely different."
I really like this path, as here quests can change the adventure path for the player and make for a much more engaging experience where your actions have a real affect on the world around you.