For me the main requirement is to pay respect to the song: in contrast to egotistical performers who frequently leave once their rendition is complete and miss other people's contributions.
The magic of a singing evening is the unexpected gem that surfaces now and then, and the thread that inspired the link from one song to the next.
True that a balance must be drawn so the audience is not swamped by long ballads, however the ability to listen to the narrative should also be encouraged.
Tolerance and respect are required: the world is too small a space otherwise!