As Drakonis says, there dont seem to be any timed quests as such. (Edit: Whoops! Except for the things Raze mentions of course - he has encyclopaedic knowledge of DD.:) ) However, there are a couple of fairly important things that do affect the timing of when you might want to complete quests.
SPOILERS BELOW
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1. Once you reach the Wastelands (after the Council of 7 business) you can't go back to the earlier maps and finish quests or explore areas.
2. When you enter Stormfist Castle you lose your teleport stones, and don't get them back until later in the game.
You can still explore the rest of the main maps after you get the stones back, but for the flow of the story it's probably a little smoother to do most of your work before going to the castle. Eg. explore the Dark Forest, and Verdistis.
Not essential to to it in any order, so it's your choice, but that's just the way it seemed more comfortable to me. <img src="/ubbthreads/images/graemlins/smile.gif" alt="" />
Good luck, it's great game and very flexible in all the ways you can approach it. <img src="/ubbthreads/images/graemlins/smile.gif" alt="" />
