Well, if it were up to me I'd put Nu-Metal under the umbrella of Metal. But, I guess it's not. When I hear Korn I think Metal, but then again I don't know much more about Nu-Metal than Korn anyway.
To be honest I don't really care what you call it, I'm not even a big fan of the style anyway.
What we need now however is a separate genre just for DragonForce. I'm a big fan of Power Metal, but not of DragonForce. They should be called Silly Metal, or Stupidly Fast Metal, or maybe WTF Is This Metal.
