I've never had any formal training, so I can't really add much to some of the above discussion, but it seems to me that timing has more to do with a "good" voice than actual tonal quality, range, etc.

Somebody mentioned several singers whose voices aren't musically good — apart from being able to stay on pitch — and I think that list could be expaned enormously. Ray Charles didn't have that good a voice if you're talking tone, but his timing was superb. Neither did a lot of the old blues men. Or, for that matter, a lot of country singers. Is Bill Monroe's voice "good"?

And I even find staying on key suspect as a criterion. Buffy St. Marie made a career of singing half a tone flat from whatever instrumentation she had.

I agree with some of the defense of technique, but I still can't stand most trained voices doing anything other than what they were trained for — which usually isn't a type of music I like. My wife is an opera fan and — when I can't avoid it — I hear quite a bit of it. She marvels at the purity, power, range, emotion, etc. of many singers, but to me they all sound (as they seem to be trained to) like they've just been given an enema in the wrong end.

(Sorry, opera fans — guess there's no accounting for taste, or lack of it.)

david