edit: This expansion took a different, lore-oriented approach to the word Legendary. This time around, you went on a questline built from the ground up around building your character as the name to know on Pandaria, with your benefactor (Wrathion) empowering you accordingly as his champion in ending the Alliance-Horde war decisively to prepare for the Burning Legion invasion he foresees in the near future. You're supposed to be the sleeping giant that gets woken up and inspires both factions to fall in line, and for what it's worth you win the respect of just about everyone on the continent except for the other faction. By the time you kill the Sha of Fear, your name's whispered among the locals and you've socketed a Sha-corrupted weapon with the very essence of the Prime Sha of Fear. By the time you strike a decisive blow in the war on Krasarang's beaches and elsewhere, you're able to further empower your Sha-corrupted weapon (or, indeed, one of the Thunder King's personal armory) with a socket that can hold nearly any gem. By the time you've killed Lei Shen and finished gathering long-forgotten relics of power from his stronghold for Wrathion, you've empowered your helm with the essence of the Black Dragonflight. By the time you give Wrathion Lei Shen's heart, and get the blessings of the Celestials, you're given a cloak representing you as the champion of Wrathion and the Celestials alike--a legend in your own right. By the time you go to end Hellscream's reign once and for all, your cloak is further empowered on the Timeless Isle to legendary levels, befitting the legend you've carved out during your time in Pandaria.
The item's not legendary because it's rare or hard to get--it's legendary because you've spent all expansion doing every tier of content, every minor story patch (and two battlegrounds that were introduced with this expansion), every 5.1 and 5.2 daily hub, earning reputation with your benefactor, proving yourself to the closest things the pandaren have to gods, and carving your name into the stars. You're a legend now, and you're rewarded accordingly. It's not a traditional approach to legendaries, but building up to one over the lifetime of an expansion with a steady stream of character upgrades culminating in a legendary item is one I'd actually like to see them continue in the next expansion.