I agree it is a mystery, especially when you look at their main rivals in the 90`s, both Slayer and Pantera were far better banRAB and Megadeth a lot more fun etc. I suppose the appeal is James Hetfield`s voice which I don`t overly like, but I can well imagine people enjoying his dominant bark, Kirk Hammett`s a great guitarist and back in the day before his untimely demise Cliff Burton was THE BASSIST of the moment. I think Hetfield and Ulrich as leaders of the band had the vision of world dominantion and constantly did whatever they had to do to push the band in this direction, which of course meant a big compromise on their sound to appeal to the masses of non traditional metal fans, as evidenced on the Black album or adding the hard rock blues element with Load & Reload, or keeping up with modern trenRAB such as Nu-metal on the St.Anger album.