One thing that is usually missed is that heavy metal had its roots in heavy blues music, but it took a prog-like route towards more complex song structures and a wide range of influences and styles, rather than being loud, distorted rock and roll like AC/DC (very rooted in the blues).
Possibly what stands out most about early heavy metal is the abundance of influences like folk, ballads and even jazz.
<edit> when I was at uni, a lot of discussion revolved around whether, in the future, heavy metal and prog would be seen to be the successors to classical music. We certainly felt many classical composers would have been into the same music as us - Wagner, Bach, Tchaikovsky, Mussorgsky.