Lighten up, these are not "rules" you have to follow, but suggestions how to get a better response from a folk-oriented audience. It is something that is also known as "feedback" - the unplugged version, so to say. If it sounded like a consultants memo, that was meant tongue-in-cheek (If you want to accuse me of something, quote G.B.Shaw: "Those who can, do - those who can`t, teach").

Of course every day and every crowd is different and what works one time doesn`t on another day. I just felt that "Act as the situation requires" doesn`t really contain helpful information. This is what you have to find out for yourself. See my # 6.

M.Ted, I don`t think I am excluding someone. My suggestions concerning instruments simply don`t apply for a-capella-singers (I like a good a-capella or sean-nos song, by the way); the suggestions about known material does not apply to ethnic/world music. If it is anounced accordingly, nobody will expect well-known stuff. Historical or nostalgical material will certainly be recognized by the audiences I am thinking about(an anouncement can be helpful here too).
As for the anecdote about "The Cure": I have seen a good few bands that thought they could do likewise - but no one in the audience shared these thoughts.
Instrumentals are also a good way to show your abilities as an instrumentalist - it is also easier to play something more demanding when you don`t have to sing at the same time.
Learning to play your instrument well helps in the long run (I remember hearing Steve Knightley saying this - just in case you want another opinion ;0)
If someone is too lazy to learn his instrument well, he should consider singing a-capella, get an accompanist or stick to reciting poetry.

Best
Ernest
P.S.: Love your pun, old hand. Do you know the song "In dead earnest" by Lee Hays?