john charles,in an earlier post said"There is hope for us all then if it is not necessary to be really good in order to make a living from folk singing."
I think that is an interesting and extraordinary statement.
I think it is necessary to attempt to be good, that involves practice and a certain amount of natural talent[ however natural talent without hard work is not enough].
listening to one self, being self critical and working on improvement are important, however I do not think it is necessary to be a virtuoso[someone who is really good].
my opinion is this it is necessary to have a varied repertoire [varied in subject matter, tempo, keys]
various accompanying instruments played to a standard of competence can add to an overall effect of variety, but are not an absolute necessity.
[Roy Harris managed to provide excellent evenings unaccompanied] abilty to communicate with[rather than to] an audience, good presentation.
it will happen occasionally [and it has occasionally happened to me] that a professional performer will be tired etc and may forget the occasional word, the professional knows how to deal with it, that is to make light of it and carry on, that is part of the art of performing.
Finally professionalism involves showing respect for material and audience, it is an insult to both.. to get so drunk that the performer is unable to deliver.