I agree with a number of posters who've pointed out that voice training alone isn't enough--you have to know what's appropriate for the music you're singing, and too many opera singers don't seem to understand that a full-on opera style doesn't work with folk music. Some few, fortunately, do get it, and they can bring something special to it.

Nobody's yet mentioned Odetta as a folk singer with classical training. You can hear that if you think about it, but because she knew what to do with it, and what not to, she just sounds like a great, great folk singer.

I've never been excited about opera per se, though some operas and singers I like very much. What I dislike about many opera singers' voices is a quality I don't have the proper technical vocabulary to describe. It sounds, not exactly strangled, but as if their voice is just too big for their their throat, and not all of it can get out, leaving some trapped inside the body. I find it uncomfortable to listen to whatever they're singing. Pavarotti, for example, had that quality, so my tolerance for him is very limited, but Domingo doesn't, and I could listen to him any time. (Can't say I recall hearing either of them attempt folk songs.)