Sorry Laura, you asked what appeared to be a simple question, but there are so many angles to this... Joe's posting above made me think of another one: The choice of material.

Some songs require power from the voice. Others need delicate weaving of notes. And others need a storytelling, "parlata"-like apoproach. Most songs need varying degrees of the three.

And so most singers, with whatever voice limitations they have, tend to gravitate towards those songs that suit their voice best. Most, though not all - I have heard some atrocious combinations of great voice/wrong song in my time.

I am told that I have a "world weary" kind of voice, which thankfully suits most of my songs. Yet, and although I adore the blues, I cannot sing them to save my life. I sound crap. Wrong kind of "world weariness", I guess...