I find there's an extreme few bands that can successfully create a truly depressive mood through metal. Bethlehem's and Abruptum's bleakest stuff can't be described as metal, only black noise or whatnot. Then your bands like Radiohead....meh, even Coldplay are more depressive than most of the dirgiest doom bands, who are often just boring.
For me, the most effective path to desolation is minimalism. Chunky distorted guitars just do not equate to sadness, no matter what they're playing or how fast.