onion, I haven't attempted (or had time) to work on a quest mechanism in a long time.
I wish I had some good tips for you. I think 95% of the work will be in planning, before you code anything. I used a diagramming app (i.e. Microsoft Visio) and a spreadsheet to list out all of the objects and events, then mapped out their interactions.
You have to consider the life of each object, and how every event could possibly affect it. The more objects and events you have, the more complex it gets.
You will also need some way to manage the state of the game, like with a finite state machine. rexrainbow has an FSM plugin, but I haven't had a chance to mess with it yet.
I wish I could be more helpful.