You could probably create a laundry list of performers with voice characteristics that would guarantee they would never perform grand opera or standard pop material, Broadway show tunes, etc. But with the right material, musicianship, charisma, stage craft and "heart," I have seen a lot of these same people move audiences to tears, laughter or deep reflection.

I believe that "integrity" is the core value. Rod Stewart comes to mind, as does Dylan...many others. Maybe it's a combination of chutzpah, confidence and a "what the hell; here I am, take it, or leave it" attitude combined with material that resonates. You know it when you see it.

I don't think you could easily define a set of standards that fit. The audience, just as a jury, is the ultimate "tryer of fact."