I really believe that we should be able to do old quests again with other classes. There are so many reasons I think this should be implemented.
The dialogue and story are fantastic. The zones and environments are beautiful. We should be able to experience this many times, instead of the brief, fleeting encounters we have now.
Quests are a great motivation in continuing to playing. Maybe, on successive runs of the quests, we could only get experience points, to prevent any possible imbalances in the economy. Also, maybe we should only be able to repeat the quests starting at level 15 or so; necessitating a bit of a grind still.
Running quests isn't even the fastest way to level up, but it's a consistent source of content. I would not mind doing the story-quest-line and side-quests 8 more times.
After finishing the story-line and playing other classes, there is no doubt that the game feels empty.
Please implement the repetition of quests.