I used to have a problem with Slayer solos, but Bathory and Possessed/Morbid Angel use similar "compositional techniques" for their solos, to the degree that, through gaining appreciation of those works, I can now stand Kerry King's wankfests!
Metal should follow from Burzum when it comes to "solos", in that they should not so much showcase ability as they should provide a lead voice that actually adds something to the music.
Another interesting example is Tom Warrior's solos on To Mega Therion: his paucity of technique renders many of his solos incredibly similar, if not identical in parts. The effect is great, though, as those sections link the various songs, allowing the album as a whole to flow better. With my own soloing (again, paucity of technique), I try to throw in tiny references to other songs (or earlier bits of the same song), for the same purpose of providing cohesion.