Eric the Streetsinger just beat me to it. The essential thing is communication. If someone understands the song and communicates that understanding to an audience, that person is a good singer/communicator/entertainer 'cos the audience will like the result.

If someone with a 'trained' (and I suppose I mean classically trained) voice attempts to sing a folk song the result usually sounds horrible, the classical training results in sticking to the dots at all costs, and the cost is usually any emotion inherent in the song. It's the same with classical musicians at a diddly ay session, they just can't do it, they generally can't seem to get involved with what is essentially living music (witness the Corrs when they claim to play trad 'folk').

A 'good' singer is someone who moves the audience (preferrably not out of the door) the quality of the voice is not crucial to this. Charles Dickens had awful hand writing but was obviously a great writer. Someone else may have good handwriting (analogous to a good voice) but be a poor writer (singer).

It ain't what you got it's what you do with it that determines the question of 'good'.

Phil.