Metal is distinguished from the other kinds of hard music in that it is Romanticist, whereas the others are Realist. This distinction comes from literature, where writers from Walter Scott to J.R.R. Tolkien to H.P. Lovecraft are classed as Romanticist, and writers such as Stephen Crane and John Steinbeck are classed as Realist. Realists concentrate on the current condition of humans, they tend to be materially and politically oriented, and they often exaggerate the bad in life. Romanticists concentrate on the ideal condition of the human spirit and tend to exaggerate the conflict between good and evil. Punk bands most readily fit into the Realist category, while European power metal bands are most obviously Romanticist. Just compare Green Day's masturbation song to Iron Maiden's "Rime of the Ancient Mariner", and you have the Realist-Romanticist distinction in a nutshell.
Romanticist writers have often been criticized for not promoting "social consciousness," while Realist writers have been lauded for their stark portrayals of the evils in society. In the same way, metal bands have been criticized for not being "socially conscious," whereas alternative bands are praised for "addressing the issues." I don't think music should be used to promote social consciousness or any other agenda, and I tend to think that such propagandizing detracts from fiction as well.
Let me just point to some of the other benefits of metal as well. Metal is cathartic. It helps us purge our wrath and fascination with evil. It also serves as an excellent vehicle for extreme emotions. Metal songs run the gamut of emotions, from giddy joy (almost anything from Helloween) to righteous anger (Anthrax, "1000 Points of Light") to adventure (Megadeth, "High Speed Dirt") to haunting fear (lots of doom and black metal). The only "emotions" metal does not portray well are complacency, boredom, and dull stupidity. Green Day does a good job of portraying those things, though. Bully for them.
Some people (e.g., "Sven" or Chris, the USENET's most loud-mouthed Satanist) contend that metal is all about deconstruction and chaos. I disagree. Some death and black metal is chaotic, but the vast majority of metal is fairly structured, though extreme. And even chaotic metal is not necessarily deconstructionist in its purpose, but perhaps cathartic instead, giving play to our nihilist urges and thus purging them.
Overall, I find metal a positive sort of expression, but one that is not for everyone. Since the music is extreme, the emotions it portrays are extreme. People who can't identify with those emotions won't enjoy metal.
Metal music also involves the listener on several levels, because there are often competing melodies and several underlying harmonies. The lead guitar and vocals will often offer equal, competing melodies, while the rhythm guitar and bass support the harmony, and the drums hold up the rhythm. Many people who have listened to metal and not liked it have told me that they "just can't comprehend it all." Metal overwhelms.
Take this as a pro-metal manifesto or just as a short analysis trying to put metal into its place in the musical spectrum. Whatever it is for you, I hope this article sheds some light on the stuff we've all been enjoying instinctively for years.