The first and most important feature of QuestHelper is the ant-trail it draws for you on your world map. Something like a distant cousin of Cartographer Routes, it will judge where you are in the world, what your current questing goals are, and then determine the shortest path around to get all your objectives accomplished and rewards obtained. The little ant-like dots will move from icon to icon on your world map, representing items you must get, or monsters you must kill, as well as where the quests can be turned in once you finish. It knows a bit about which areas of the map are impassible, and it will usually point you to waypoints you need to follow to get around obstacles. If you are questing together with other players using QuestHelper, the addon will take into account everyone's quests and find the shortest path to get all of them finished together.
If you have Cartographer installed, QuestHelper will even use that addon's "waypoint" features to show you a spiffy 3D arrow so that you can get a clearer bearing on which direction you should be moving in; the arrow even changes color depending on how close you are to your objective, and it will tell you about how long getting there should take. Even if you don't use Cartographer, though, there's a little pulsing arrow on your minimap which can help guide you as well.
As I mentioned before, QuestHelper isn't going to level your character up for you. It knows a lot about quests, but it doesn't know everything. It won't help you pick up quests to begin with, for instance, only guide you to complete them once they're in your log. As I tested it out for myself, there were times when I knew better than the addon and did my own thing rather than follow the little ants on my map (but then again, there were other times when "my own thing" actually ended up being more trouble than I had realized). QuestHelper is also still in beta, so although it works pretty well out of the box, its command-based user interface may take some getting used to if you want to play with the settings.
Overall, however, I found QuestHelper to be a definite addition to any good list of recommended addons -- even more useful than LightHeaded if one had to choose between the two. Still, even with QuestHelper pointing the way, LightHeaded can come in handy when the actual task you must complete in the quest requires a bit of strategy or finesse, since advice from other players rarely goes amiss. You can download QuestHelper at Curse.com, and keep an eye on it there for future updates.
[Thanks to reader Mushoo, for the tip!]