Aeus says it's a good idea that just hasn't been acted on yet, but I'll add my voice to the crowd asking for action. Before, when lower level quests didn't grant XP or rep, this wasn't that big a deal-- if you were going back to do a certain low level quest (for lore or completion purposes), you probably knew where it was anyway. But since reputation has been buffed on lowbie quests, there's value in knowing where quests are. And there's really no reason for Blizzard to hide that info, anyway-- the quests are viewable right after you talk to the questgiver, so why not throw a exclamation point up there?
The only question, then, is what color. Later in the thread, Readbeard comes up with a whole list of exclamation point colors-- green, yellow, orange and red for quest difficulty, and black and gray for pre- or post-level status. As good an idea as that sounds to us WoW veterans, my guess is that Blizzard won't vibe with it-- explaining all the different colored exclamation points to a new player might be a little tough. But still, a toggle-able lowbie quest exclamation should be easy to implement, and would help those folks headed back to the starter areas to pick up experience, rep, or whatever else they want.