More thoughts.....a couple of singers that can make a good song great (and sadly, more often, an average song very listenable) for me are Lou Barlow and Jeff Tweedy.

And Kyle, re Malkmus (from Pavement....Mike, I assume you know this but just in case) I think it is his voice that elevates the songs. It works so in tandem with the shambolic lo-fi-ness in their earlier records that it is hard to separate the two. But it's a remarkably expressive instrument in its own right.