The riff. It's everything to metal. You can strip away pretty much any other element that has come to define the genre over the years - haunting atmospherics, scorching wails and guttural screams, blastbeats, basslines and a ton of denim and leather - but it all comes back to the riff.
Black Sabbath, the first true heavy metal band, were powered by the bowel-shaking, devilishly down-tuned riffs of Tony Iommi, metal's first undisputed guitar god.
