[!--QuoteBegin-macunaima+Jan 19 2006, 02:35 AM--][div class=\'quotetop\']QUOTE(macunaima @ Jan 19 2006, 02:35 AM)[/div][div class=\'quotemain\'][!--QuoteEBegin--]That's a little bit helpful. But I always heard Helloween and Blind Guardian referred to as
"power metal." So what's the difference? More to the point: is there really any point to such fine distinctions on whose application no one can seem to agree? [!--emo&:unsure:--][img src=\'style_emoticons/[#EMO_DIR#]/unsure.gif\' border=\'0\' style=\'vertical-align:middle\' alt=\'unsure.gif\' /][!--endemo--]
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I've had the same problem. but taking into account that (as someone said above) Metallica has been labeled more than once as speed metal, they are not power metal. Power Metal tends to be upbeat with happy or positive lyrics (inspirational) and VERY fantasy based (hammerfall, blind guardian, Kamelot) and very catchy. I wouldn't say Metallica has "catchy" songs (prior to the black album anyway), they are just good with intricate and fast solos.
So to answer your question, helloween and BG and the others mentioned are Power Metal because of their fantasy based, upbeat music and "happy" lyrics. Keep in mind this is generalizing as I know the lyrics aren't always exactly "happy" but the rest tends to hold true.
The idea of sub-genres helps people (I think) find bands they will like. For example: Therion and Children of Bodom are both "Heavy Metal" However I love Therion but don't care much for Children of Bodom. I want to find more bands like Therion and don't even want to bother looking for bands like CoB. So I look for "Symphonic", "Melodic", "Power" Metal bands and shy away from "Black", "Death", "Gothic" Metal bands. That is the only real purpose I see for sub-genres.