Metal, just like every other form of music, just like every other fine art, et cetera down the line, is subject to the institutional framework in which it is viewed, and is in essence a very historical construct. It continually changes, evolves, and adapts, so any rigid, static definition of 'Metal' that can't account for this can't possibly be valid. A band like Deathspell Omega or Drudkh is no less of a Metal band than is Judas Priest or Black Sabbath, though they probably would not have been viewed as such if they were the ones to have been around in the 70s. In fact, their very existence depends upon the historical progression of the genre, so a definition of Metal, if such a feat is even achievable, would have to be open and adaptive to be able to incorporate further progressions in the genre.
Also, I strongly agree with and support Cythraul's post regarding expression. It is very important to view music, and art in general, as expression, and the artist's expression and interpretation very much is at the forefront in interpreting that artist's work.