The Most Brutal Song Ever

I think the biggest gap to bridge for most people going from pop music to extreme metal is understanding vocals being treated as another instrument in the mix instead of the main focus with instruments merely being there for backup.

YES. That is beautifully put. It's not exactly transcending some boundary and gaining access to a higher plane, but it sort of is. I've got a friend sitting on the fence with that sort of thing...he loves the instrumentals in Emperor/Dimmu-esque black metal but he can't stomach the vocals. I'm trying to push him over the top using the exact same logic - the vocals fade into the song and work alongside the instruments to make a whole.
 
I can understand someone not liking the vocals in extreme metal. They are purposefully harsh. The problem is when they listen to the vocals more than the other instruments, expecting the vocals rather than the instrumentals to be the leading part of the song.