In many fantasy realms and games from Middle Earth to games like Shining Force, Mithril has held a place as a much coveted and legendary metal from which weapons and armor unparalleled could be forged. Even in original WoW, Mithril was good but not great.
Mithril, also called Truesilver (another mundane, if still rare metal), has never been anything in Azeroth except for a metal of higher quality than iron but lower quality than fel iron.
In fact, most Dwarven civilizations are portrayed to have a passion for this metal. From Fall to Earth (or whatever that Civ 4 mod is called) to Dwarf Fortress, Mithril has a special place in a Dwarf's heart. The Dwarves of Azeroth, if their city names are an indicator, have a stronger attachment to iron. . .
I am not putting Blizzard down for departing from established lore. I wish only that mithril had a more special place in the grand scheme of things. . . .