Let's begin by discussing genres. I enjoy several of them.
I'm probably the least "Metal" person you could ever meet—I'm rather placid and slight of frame—but god, I love it. I crave it. Being a surburban white boy, I'm genetically predisposed to metal, I suppose. I'm addicted to it—it's like drugs or porn. You start soft (in my case, Kiss, who are more bubblegummy than soft, I guess), then you slowly become aware of harder, stronger stuff. Before you know it, the soft stuff doesn't do it for you anymore, and, in search of a more powerful jolt, you end up in dark, obscure territories. That's what happened to me in the '80s, and I still haven't emerged. Motörhead, Metallica, Slayer, then hordes of speedy European bands kept me enthused through the decade, and I'm now at the point where nothing much sounds new to me on the extreme end of the metal scale. What I am enthused about, however, is the slew of excellent bands from Europe (mainly Scandinavia) who've grown out of the "extreme for extreme's sake" ethos and incorporated influences outside of metal into their sound. I'm thinking of groups like Ulver, In the Woods, Opeth, Anathema, and so on...not exactly household names. All of them are more inclined to sound like Massive Attack, Pink Floyd, or Nick Drake than, say, Megadeth.
Anyway here's a site with a gleefully academic slant on all things metallic and extreme: The Dark Legions Archive
Some sample text: "Black metal is dark and fast music using melodic development to express its themes. Of all the metals, this is the most communicative with the modern listener, expressing nihilism and a heroic anti-social assertion of the self. Evolving simultaneously with death metal, this genre includes all of the technique and rhythmic intensity of the former with more emotive and comprehensible poetic communication within the music."