Wow this is a tough thread. Hum....
Appeaarence definatly has a part in this question. To me, the performer has to look like they're having a good time and know what they're doing.
Originality is also something to me. It's probably not as important in this category as it is in other aspects of performing, but still, if you sound just like someone else, to me that's a "turn off" (if you will).
I agree that the performers should be their best every night, but if they're having an off night, so be it. They're people too, and they need to rest. Just because the person is having an off night doesn't mean that they're going to be a terrible performer for the rest of the show.
~*Christa*~
"Don't ya wanna be the life of the party?" Idina Menzel, THE WILD PARTY