Well, I just posted this in a response to another thread, but decided it would be worth posting as a top level.
Very simple. It's along the same lines as GetCompletedQuests(), except it returns a list of uncompleted quests for which you have met the pre-requisites. It doesn't tel you anything about them, not where to get them, not what the pre-reqs are, not why you didn't meet other ones. Just a list of ids of quests you could get if you spoke to the quest giver.
This would make the quest assistance mods much, much cleaner, and really wouldn't provide any information to the players/mods that shouldn't be available. I can quickly hand-scan quests in QuestHelper and realize that "Water Totem" quests are not available for my Hunter, but it's less obvious that I haven't done the pre-req "go help SomeDude" so I go to SomeDude, and later find out I had to stop by Orgrimmar and pick up a quest to start the chain...
Seems like it'd make life easier and nicer, without providing anything that isn't somewhat calculateable but is super annoying to figure out
Probably won't happen, as it's already easily doable by datamining a complete list of quest IDs available for your class/race (eg. from wowhead) and comparing it against the list returned by GetCompletedQuests() to determine which quests you haven't done yet. The reason GetCompletedQuests is so useful is that it provides information that can't be obtained through the API or through any kind of datamining.