I've always wondered, what is the medium of metal guitar playing, like what band or song really stands in the middle of average?

Yes, poop.
average is around the skill of most metallica songs if that helps

of course solos are advanced
I would say if you can play the rythm parts for Metallica's Black Alum WELL, you are "average." If you can play the leads WELL, you are above average. While they're not that complicated, they do take some dexterity.
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cmon guys avrage is all the playing that is not well played, thats all.
you can play the easiest song in the world, and play it average
Yeah if you can play most of Metallica's rhythm parts than I'd say your at least average. I'm not the best soloist, but when I put a little determination into learning a song I'll be able to do pretty much any easy to moderately hard solo pretty decently.

Then again, a lot of people can't even play the simplest of songs. I've always thought as long as you can play at least one song really well than I'd call you a good guitarist no matter how easy the song was. I still stand by that statement .
I think he means something like what should your playing be like to be considered beginner, intermediate, or advanced.