Originally Posted by Forward To Death

There is a lot of metal that sounds like rock musically though, like power metal/hair metal bands, the latter often being considered a rock genre.

I think what separates metal from rock is the more abrasive themes, it's almost like punk in that it almost has an ideology attached to it. Some hair metal/nu metal bands for example are accepted by metalheads, while others are considered rock bands.

Personally, I just know metal when I hear it. I can't describe it perfectly, I can generalize that it's typically louder/faster than rock, but that's not always the case.