Ken, I think your assessment is totally accurate. McPhee's style is a throwback to an earlier time, as her complete comfort singing Cole Porter demonstrates. Her style, looks and voice are not good fits for the pop and pop-rock music of today. She's a throwback to the glamorous female stars of the 1940's. Plus, she started out in musical theater before auditioning for Idol, and she could never quite get the theater out of her system while on Idol. That's what caused her to lose the title, in my opinion.
If her acting turn on CSI: NY -- as a murderer, no less -- and in that upcoming film, show that she has real talent, I believe Kat is on her way to big-time stardom. The only thing she has to demonstrate is acting talent. She has everything else necessary to be a big-time leading lady.
I'd add song selection to the list -- she kept trying to be a Mariah/Whitney style "diva", but she is much better suited to a more husky/vamp style. Ertha Kitt or even Marilyn Monroe would have been better role models for her.