Quests
Blizzard has stated that questing in Diablo III is something they're adding extra polish to. In addition to the main storyline, each class will have their own backstories and optional questlines throughout the game.

Unfortunately that's about as much information as they've given us. Add a bookmark and check back here later on and as we'll have a full array of features for quests to supplement your gameplay in Diablo III.
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