I don't know my metal well enough to say anything definitively, but I believe Metallica might just be an advanced heavy-metal band. I think that "Master of Puppets" is really good, and a lot of people I know are very enthusiastic about their earlier stuff. However, with stuff like "Enter Sandman," they moved into the category of "world's most evil pop band" (thanks, Britt). Of course, being a metal band, they had to have short hair, rather than long hair, to show that they had advanced. Could it be that their decision to file suits against file sharers was an advanced move? I think they also sued someone who named a rose metallica, which was based on it's latin description (or something like that) and had nothing at all to do with the band. And now they've done a documentary about themselves, including rehab and therapy sessions (not very metal), which seems pretty advanced. But were they good enough in the beginning to qualify?
Something to think about.