Ah, the eternal question.

Well, the sound is different really - between commercial & non-commercial hardrock/metal. Which is why some music ends up being available to the larger masses, it's appeal is easier to confront but it isn't necessarily the main cup of tea for "real" metalheads (with "real" I mean metalheads who have been around for a long time). I'm not trying to say that commercial music is worse just because it is commercial, the sound is just different. I generally prefer bands which aren't as easily approached, so to speak. It's quite hard to explain. But of course, if I ever come across a commercial band which I find great - I don't mind if it is commercial. Only posers & younger wannabe metalheads do that. As you say, music is always music. Also, one of my favorite bands is Nightwish - which today is rendered a commercial band (though it wasn't when I started listening to them). They are still very good however.