In my mind (no doubt incomplete and definitely overlapping):
Metal is various out of: Loud, Screaming, Negative emotions, Orcs, Wizards, Stoned, Warfare, Darkness, Demons, Leather, Howling guitars, Melodic base, lots of hair in the past, more of a British feel
Heavy rock veers towards: Loud, Shouting, shagging, party feel, girls, cars, pissed, chugging guitars, rhythmic base, denim, big hair, more of an American feel