In Mistborn, metal powers Allomancy and cannot be seen by Ruin.
In Stormlight, the currency is Spheres, a kind of metal that also holds stormlight.
In Elantris, metal plates are used to teleport around the city (used by Elantrians).
In the Emperor's Soul, there is soulstone, presumably some kind of metal.
This one's kind of obvious, but Nightblood is made of metal.
Steelheart: He can turn things to steel.
So why do you think metal is of import in almost every one of Brandon's books? Is this just a fad of his or is it actually important to the Cosmere?
(After all...shards are made of metal...)