I've been thinking of quest-threads for a long time--despite seeing them in action, despite contributing to them, despite reading them, I still don't actually know the "theory" behind a quest-thread's development.

What do you think makes a good quest-thread good?
What makes a quest-thread stumble?
How do you think is a good way to plan or GM a quest-thread? Is planning things out ahead of time or improvising with a vague outline better?

I know shit all and would like to know more.