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.