Just a point of specification on the subject of the classification of metal:
Metal in it's extreme variety (From Iron Maiden to Cannibal Corpse (And yes, Iron Maiden can be considered a part of the NWBHM (New Wave of British Heavy Metal (Yay, 4 "()"-s)))) is categorized very differently. This also caused by the 80's boom of heavy metal where people really just listened to the music because it was popular and therefore alot of the categorization has been lost there. To this day each individual can freely categorize heavy metal up to the point of calling Hard Rock, Punk and Glamrock subgenres of metal (as they all have roots reaching into metal)
However, generally, one can still say that from the early metal bands (Blue Cheer, Black Sabbath), Metal has gone to 3 large types: Hard-rock-style bands that can really be categorized to both rock and metal (Black Sabbath); Operatic metal that takes inspiration from classical music (Iron Maiden); And ofcourse extreme metal that takes up the most space in the chart (Slayer).
These 3 divide into hundreds of subgenres from Progressive Metal to Thrash Metal, From NWBHM to Norwegian Black Metal. Some are almost christian (Black Sabbath (yes, Black Sabbath)), Others burn churces (Gorgoroth, anyone?).
At first my band of choice was Metallice, but after a while i needed something a bit more... heavier. However, i find good riffs (Rob Zombie - Superbeast), masterful solos (Iron Maiden - Pilgrim) and powerful drums (Disturbed - Down With The Sickness) equally entertaining. Though i don't really like the band, the best definition for my taste would be Cannibal Corpse - Infinite Misery (Put away your prejudice, it's an instrumental ).
Yeah, that turned out to be a slightly longer post than i expected...