Well, metal requires alot of technical skill, as if it were a Bach Sonata or any other difficult work of classical music. Not just any noob can be good at metal. I especially find the connection in stuff like Opeth, Ensiferium, Alestorm, Dimmu Borgir, Ect. In fact, if you need to see a huge metal/classical correlation example, look up Progenies of the Great Apocalypse by Dimmu Borgir, featuring the Prague Symphony Orchestra. The form of good metal is often based off of lots of difficult concepts, involving recapitulating the melody, and using chordal progression modes, commonly with phases that take up 4 or 8 bars. This is excluding our math metal of groups like Meshuggah, but you get the basic idea.
Metal is most commonly in a minor key in European metal, because it is based on folk roots. The cousin of the minor scale, the blues scale, is found in a lot of the American heavy metal (God forbid I mention this band on this site, but Avenged Sevenfold comes to mind as ones who use lots of blues scales.) We also have the punk influence in American metal, so I would say that European metal had this closer correlation. Groups like Nile use harmonic minor scales, which is seen just as often in classical composers, such as Georges Bizet.
One last thing that makes metal very similar is that it is one of the frontiers of the progression of all music. One hundred years ago, Maurice Ravel was writing piano pieces that got him booed off of stages, and his music was considered completely blasphemous. Fast forward 70 years or so and you’ve got groups like Maiden, and then listen to groups out now like Napalm Death. Metal is where our music is moving, and it keeps the “heavy of the heavy” alive, just like Bach did back in 1700.
Does this all make any more sense? I explain this theory to alot of people all the time.