Something my roommate and I started talking about while listening to Iron Maiden and Metallica this morning; at what point in metal did the harmonizing guitar riffs become popular?
A great example of this is Iron Maiden's The Trooper
It didn't originate in metal, but showed up in rock in the 70s with bands like Boston and the Allman Brothers. I don't know if I can point to one bands that did it first, but it gained popularity then.