I'm not going to have an opinion on bossa nova ofcourse, since it is a culture in itself. And well, you really don't argue about stuff like that. Besides, I've already made comments about it on a previous post.
I just want to describe Sitti's voice as gentle and
precise. The reason I say this is because most female solo artists who get to be popular in radio are power divas, i.e. those that can hit high octaves and get away with it (Nina, Rachelle Ann Go, etc).
Either that or that certain 'character' in your voice (Juris of MYMP, Arre of UDD).
Sitti's voice isn't all that spectacular. But each word utterance is delivered in a clear, soft manner. You could say she did train a lot (compare: Enya). It could be she is packaged that way, or the style of music makes it so. The thing is, I particularly notice those kinds of things. With other artists, you could simply live with the tune, but in her case, you'd find yourself absorbed in the lyrics.
Probably just me :)