<i>Heavy metal is (you have to admit that) a pretty rigidly codified type of music. It requires a certain set of instruments, with few variations</i><p>Nothing could be further from the truth. For example, the OP itself talked about &quot;folk metal&quot;, a sub-genre that uses traditional themes, musical styles, and instruments.<p><i>It sounds in a very recognizable way, so it&#x27;s rather easy to classify songs that fall into it.</i><p>Again, you&#x27;ve missed out (and even claimed that it never happened) a HUGE amount of evolution. Metal fans love to bicker and debate, and you&#x27;ll see, for example, discussions over whether the evolved Opeth, that eschews their older death metal trappings, still counts as metal at all. In the proggy sub-genres that I enjoy, there&#x27;s ample debate over whether a particular band or song is prog <i>metal</i> or &quot;just&quot; prog rock.<p>In its infancy, much of metal was dismissed as being stupid, three-chord performances. It&#x27;s evolved so that today, it&#x27;s undoubtedly the most technically demanding genre within the entire poo &amp; rock oeuvre. Bands like Meshuggah, or the whole math-metal sub-genre, are performing music so technically demanding that no high school garage band is going to get near it.<p>In its expansion into these prog, technical, death, extreme, folk, etc directions, there is no doubt that the variety of expression covered by metal today is many orders of magnitude greater than in its younger days.