Top 10 Most Typical Features of Metal Music

Not all metal songs are extremely fast but in general metal tends to be very fast. The average speed in metal is the highest available - we all know metal is the fastest music genre.
(I don't count electronic/techno music where the fast beats are achieved by technology and not by extraordinary human skills).

I put metal solos higher than riffs because there are pretty good riffs in hard rock and blues but metal guitar solos are unmatched in terms of complexity, speed, length and technicality.

But the heart and soul are painfully missing. Complexity for its own sake is no achievement.

Some subgenres are less aggressive but aggression is typically present in metal songs, including aggressive vocals.

I can't imagine metal music without double bass drumming. This is such a beautiful thing. Moreover, it's widely spread in metal - out of the major metal bands only Iron Maiden didn't use double bass drumming. But probably 99% of all metal bands have used double bass, including the 70s proto-metal band Deep Purple.

This element becomes clear when you analyse metal music deeper. It's easy to identify speed, aggression, loudness, etc., but you have to pay special attention to what is behind metal music, i.e. what is its 'structural foundation'.
In some sense Classical Influence is the most important feature of metal music and is used to differentiate metal from hard rock, punk rock, etc. This explains why metal goes with classical - as some correctly say, metal is classical music in the era of electricity.

I made several lists about this with details and samples:
1) Top 10 Iconic Metal Songs with Clear Classical Influences (Except Symphonic and Neo-Classical Metal)
2) Top 10 Metal Songs that Include Classical Music Instruments
3) Top 10 Classical Style Cover Versions of Metal Songs
4) Top 10 Metal Musicians Who Were Probably Classical Musicians in Their Previous Lives

So basically, metal is when classical music meets rock and roll?

Metal music is powerful and needs powerful vocals because all elements in a music genre have to be matched.

Metal riffs are usually faster, more complex and more melodic than your average rock riff. Very often metal riffs are stand-alone melodies (The Trooper by Iron Maiden). Metal riffs have faster chord progressions than rock, including punk rock, and this makes metal riffs harder to play but at the same time it makes them sound more melodic.
You'll get the difference from these videos by Charlie Parra del Riego - watch carefully his hands while he plays punk and metal:
Punk vs Metal
Punk VS Metal 2

What is expected from metal musicians? - to demonstrate skills that are above the average in a certain era/decade. Which they do.
I would quote a non-metal musician who likes metal and listens to metal - Demi Lovato:
"Metal is so completely different from anything else. You've got vocals that I can't do, the double bass drums, the incredible guitar solos... every part of the band does more difficult stuff than your average rock band does. I listen to these bands, and I wish I could do all of that, but I can't."
She obviously listened to metal very carefully. And yes, she was honest and unbiased that makes me admire her personality because she's in a genre that is very different. Moreover, unlike many people, she wasn't jealous.

Metal genre is known for these things and it's easy to prove.