Exactly. Listen to Rodrigo & Gabriela cover Orion. It's all acoustic, and still definitely metal. While Three Days Grace, for instance, no matter how much distortion they put on their guitars, will never be metal.
My thoughts exactly. I'm going to have to agree with Sami Philadelph that "Everything is contextual, and, 'you'll know it when you hear it.'"
There are certain characteristics that seem present across much metal music but then there are usually exceptions to anything. The definition should be flexible, I think. Perhaps there are certain qualities to metal that any sort of music can have in any combination to be considered metal, but I cannot confidently make such a list of defining qualities. I think presentation is key. A certain level of self-awareness and self-image may be necessary for any band or song to be "metal." But there may also be some bands that are usually labelled as metal and they don't think of themselves as such. I'm not entirely sure on that.
My take would be the Distortion presence kind of Put Hard Rock
on the Table but then when does it become Metal ?
When it captures the dark romantic spirit and transcends the humanistic and individualistic perspective that pervades our time. At that point, it can even drop the distortion and still be metal.