Clive's right on, not surprisingly. Diana is unseasoned as a singer and it shows badly. A big voice isn't enough and sometimes just isn't necessary. Diana is not unique by any stretch but Fantasia is nothing like the current scene or even really like past musical personas. A times there's a hint of Macy Gray in her voice but that seems to stem from her ghetto country (or whoever she termed it) roots. She's clearly a bigger talent and surprisingly broad in expressing her emotional range in song. She's as convincing doing Summertime as doing A Fool In Love.
Diana can't sell it which makes her on the level of manufactured pop princesses, she's perhaps further handicapped by her Britney-ish vocal style. She's out classed by, say, Christina since Christina has greater depth and is far more seasoned. And, unfortunately for Diana, *that's* what she'll be competing with out in record stores if she wins.
Jasmine doesn't know who she is vocally and doesn't know how to use her voice so clearly she's not ready to win and step into the supposed roll of "superstar" that the Idol winner takes on.