Vocalists overly impressed by their own voice
I'm listening to Jane Monheit at the moment, someone I'd seen favorably compared to Diana Krall.
She has a nice voice, and some touching songs, but she seems incredibly self indulgent at times, letting herself try and touch all bases in a single song in a way that sounds to me like it harms, rather than helps her effort to create an effective song.
Clay Aiken American Idol type singers are well deserving of the parody pieces they inspire, IMO. Just because you can make your voice do certain things doesn't mean you ought to.
Celine Dion and Whitney Houston are a seemingly two more singers who won't ever find a bigger fan of their 'talent' than they themselves are.