The Burning Crusade is heralded as the golden age of World of Warcraft, a time when the genre that WoW perfected hit a beautiful peak, a wonderful balance between difficulty and accessibility, that appealed to the hardcore MMO veterans who dominated the genre until the Wrath of the Lich King days. Others hated it entirely. No matter which way you lean, it's clear that The Burning Crusade had strong art direction, with otherworldly weapons and armor juxtaposed against traditional fantasy tropes. The fact that Burning Crusade items are the most popular right now does not surprise me in the least.
Check out all of the statistics, as well as each class' top five transmogged weapons, over at MMO-Champion.