actually i noticed that if you've done the King Leoric quest you always get the Lachdanan quest if you move further into the game without restarting. didnt notice if some other quests led to others. I always thought of a random quest pool that picked quests on every game start without much reason.
Blizzard took some of the Diablo and Diablo II music themes for dark places in WoW: Burning Crusade (Shadow Tomb, Shadow Labyrinths etc). Could be a hint (or not) for the ones carefull enough to notice. Cant wait for this years announcement, i wanna know if we'll get a Diablo III and when!