Based on the player's level range. As far as I know there's no way to get the list of completed quests so this would require the use of savedvariables.

Going to see if I can make a database for this.

Closing this, as it already works through our quest hash system.

There are addons recording you quest history.

You could write your own db as well. Shouldn't be that hard.

This is already in the making, actually. But if you can point us towards an addon that records the quest history, that'd be very helpful.

right, QuestHistory:
http://addons.us.to/addon/questhistory
http://wow.curseforge.com/addons/project-50/
http://wow.curseforge.com/addons/project-1307/
http://wow.curseforge.com/addons/project-500/
http://wow.curseforge.com/addons/quest-history_felstone/

QuestHistoryMap (couldn't find a webpage, so I uploaded it):
http://www21.zippyshare.com/v/r3E7fX7p/file.html

:o thanks for that, it might still be helpful only I JUST finished creating a system for this lol. It's based on the new quest hash system (adler32 checksums of the quest name + objective text + level).

Taking the easy way out and just doing something along the lines of SavedVariable->SeenQuests and SeenQuests[hash] = time(), then just SeenQuests[hash] == nil to check. As far as I can tell itt'l be the least processing intensive way of doing it (important because these checks are done often and in large batches)

Also, on the topic of available quests, the function is added as of 4706509 but it does not yet exclude quests you already have or have completed. (and currently has items/objects that start quests disabled)