That's right, you can now get a list of every quest your character has completed. Testers on the MMO Champion forums report that it's only the quest IDs themselves that get sent, but addons such as Lightheaded and Questhelper, one would assume, can probably take care of the rest as long as they get those quest IDs.
The ability to recall completed quests should be a good boon to mod makers and questers. Those looking to grab Loremaster will appreciate being able to figure out where to head next, and those that have had a handful of quests left for the title and no idea where to find them will finally find some relief. Here's to all the mod authors already working hard on using these new commands to get us squared away in Patch 3.3. Many a Loremaster will owe you a debt come Cataclysm.