I think putting some sort of barrier between metal and "normal" popular music -- avant-rock and the like -- is a bit ridiculous. What makes Darkthrone so much more special than, say, Magma?
I think it makes sense to describe what makes them different. I don't know much about Magma, but Darkthrone were in their best releases able to capture a sense of structure and change in emotion that is not matched in popular music.
There may be exceptions but, in all forms of design, you have to look at the intent. Pop music is there to make people feel content with what they have while showing them "the other side" that really isn't all that different.
Progressive rock has some exceptions, but compared to the Romanticist articulation of metal, it's primitive, even the best stuff, like Yes and Camel and King Crimson. Metal takes music a step further back toward reality, which has always been and always will be, and connects us to the similar eternity of transcendental thought.
We can also point out metal bands like Cannibal Corpse that don't fit this pattern, but then again, I'd call them pop music masquerading as metal.
Once we get past the bullshit that bad use of language thrusts upon us, we can see that like in literature, different forms of music are structured around different ideas and manifest those ideas in their form and content.
Upon that alone I say the difference is fundamental. How else can one tell in an instant that Opeth doesn't get it and "Under a Funeral Moon" does?