A query on the forums for what the longest questline in the game might be brings up this amazing page over on WoW Wiki-- a great collection of "long, exceptional, or hidden questlines" in the World of Warcraft. We've covered pretty much all of these quests before (including this cool compendium of quests you've got to do if you haven't done yet at 70), but this is a great and well-written guide to (mostly) higher level quests that have to be done to be believed.
As for the actual longest questline, it appears to be the "Fallen Hero of the Horde" line (that Alliance can do as well)-- it starts around level 50, and rolls all the way around the world and up to level 60. But I actually agree with some of the other posters in that thread-- while Fallen Hero is actually a great quest, the longest, most annoying questline in the game is probably the Dungeon Set 2 quests (also known as the Tier 0.5) armor. Back in the days of 60, Blizzard wanted to give players a way to obtain Epic armor without raiding, so they invented a long, grueling questline (with multiple branching paths) that would let you upgrade your first set (Tier 0) armor, piece by piece, into an Epic, eight piece set. I never made it any farther than the bracers, although I know for a fact that lots of folks got the whole thing done (and this was after completing the eight piece Tier 0 set in the first place). As long as the Fallen Hero questline is, I believe the Tier 0.5 questline wins, if only because it seems so much longer.