Given that after level 80, experience turns into gold, the return on time invested with leftover questing is awesome. You can pick up over three thousand gold easily just by clearing out the zones you haven't hit hard, and by vendoring off the quest rewards that you get for completing the quests, you can pick up even more. Sure, some folks will have AH schemes that will bring in more money, but Blizzard has done their darndest to make sure there's money in them there questgivers, so if you're slouching around at 80 wondering what to do, go finish up your quests.
And of course if you really have finished up all of the quests in the game (and seen all the amazing storylines and character development that go along with doing so), then there's always daily quests to work on. While they won't pay out quite as much as one-time quests, when you break down the time you invest versus the gold you get out of it, they're often the best way to cash in your playtime as well.