IsQuestCompleted(QuestID)
 

It would be really awesome if we had a function that told us our status on a particular QuestID. It could be as simple as returning 1 if we've completed it and nil if we haven't, or it could return yet another value if we have it in our logs at the moment.

There is so much awesome stuff we could do with this, and I know they keep that info on the server. Perhaps you're concerned that it would take too much bandwidth to download all that data to the client on every login. If so, you could make it query the server each time (like /who).

I'm like you, I think that bandwidth is an issue, but it could be partially overcome by returning a compressed link, like they do for profession links.

That said, I'm not sure they want to allow this information to be available design-wise, but I'd love to see this, would be great for addons like Carbonite & TourGuide.