Marje:
For what it's worth: the worst performers I can think of show no sign of nerves. They're oblivious to the sound they make, the sense of the words, the nuances of the tune, and the yawns of the audience, because they don't really care about any of it. On the other hand, people who are visibly (and audibly)nervous are often great communicators of the song despite the nerves.

Well, with respect to your comments, Marje, I think this is a bit of a mixed bag. I've seen some excruciating performances from very nervous, totally unprepared floor singers - and similarly excruciating performances from those "nerveless" performers you so eloquently describe (and I've seen them too). The best performers, in my humble view, know their material thoroughly, sing and play within their competence, and are confident enough to be able to communicate with the audience and to pick up on feedback from the audience. I they're talented as well - a bonus!