To me, it is the ability to make the song the focus of the performance.

I know musicians who are immensely talented, but who 'seem' to try to use the song to showcase themselves.

*IF* the song is done well, it will always be clear that "X" performer is someone who brings songs to life. Gordon Bok is one who comes to mind immediately. You KNOW it is Bok singing, but it is the song that is the star. To me, it was often Johnny Cash who was the star, even though he brought a lot to the song.

   I think of Jeannie Robertson and her powerful renditions of many ballads. You know who is singing, but her presentation makes the story real.

This reflexive quality of raising one's own stature BY not 'imposing' one's persona onto the song is, to me, a subtle mark of greatness.