This may not be very helpful, but I often hear singers who sound to me like they're actors/actresses playing a singer. They may have good voices but they seem to be acting out their image of themselves as a singer rather than sharing the song with the listener.
I'd include in this category a couple of singers who are very popular at the moment, so clearly this is completely subjective.

Eliza Carthy is, to me, the most heartfelt, genuine singer currently performing. She sings with love and she sings straight to the listener.