I think it's also the attitude of heavy metal, fantasy influences aside. Ozzy, Iommi, Ward, and Butler came from many of the same musical roots and substances as Jimi Hendrix (I used to have a bootleg of them playing 'spoonful', from before they were Black Sabbath), but then went tougher and added in some horror. Creedence did 'Fortunate Son', Black Sabbath did 'Children of the Grave'.
D&D is a mish-mash of all kinds of fantasy influences, so you get restless adventurers, heroic barbarians, cosmic horror, and kicking cosmic horrors in the face. When you look at a mish-mash of heavy metal, with Black Sabbath, AC/DC, Pantera, Metallica, Judas Priest, et all, the feeling you get is a rejection of complacency, an ode to the primal manliness, messages of a dark and scary world and the badassness of the individual. Even if Dragonforce had never existed, I could tell that these things go together.