Several threads here (Cannibal Corpse, Deicide, the various Venom arguments, and the Swedish DM thread in particular) have focused my interest on the question of abstraction in metal. It seems to me that key factor separating the classic albums that emerged between say 1988-1995 and both the first couple of generations of heavy metal and most of what has appeared since is the level of abstraction the great classics achieved.
The great inherent weakness of rock based metal was always its insipid literalism. With bands like Venom, Metallica, Iron Maiden, Cannibal Corpse, Cradle of Filth etc., neither music nor concept leaves much to the imagination. Such bands fill their musical spaces with the utterly predictable and their conceptual spaces with lyrics that are painfully self-explanatory. Metaphor and other more oblique approaches to communication are lacking on every level.
In contrast, death metal bands like At the Gates, Incantation, Deicide, Dismember, Therion, Atheist and Demilich introduced music that worked almost as if according to dream logic, as if calculated to leave an ambivalent interpretive space. Among such bands, the juxtaposition of seemingly opposing elements (consonance and dissonance, blasting and doomy passages, violence and beauty) served to create an ambiguous sensibility that embodied both the dissolution of the modern age and the haunting possibility of rebirth.
In black metal, this tendency was even more highly developed, with many bands projecting their music almost entirely into intellectual spaces defined by ideal rather than by fidelity to the current historical moment. Some bands did so through an embrace of the heroic past (Bathory, Burzum, Graveland and Enslaved), while others set their music in worlds that exist only in the mind (Immortal and Summoning).