I think what Joni meant by "unambitious" is that it's not trying to be something it's not, intentionally trying to rouse a reaction by infusing the voice with mock emotion or pretty trills. Celine and Streisand really put on a huge vocal show, a sound that is quite different from their speaking voices. There is a bit of pretense to it, I guess - at least in Joni's view.
I recall another interview where she said she hated white singers who try to sound like black singers and put all those "negro affectations" in their vocals. I guess she just likes very plain voices that are not particularly grand...or ambitious! -Andrew Yahoo! Mail Plus - Powerful. Affordable. Sign up now