Hilary Mantel has it spot on, for me:
Antoinette as a royal consort was a gliding, smiling disaster, much like Diana in another time and another country. But Kate Middleton, as she was, appeared to have been designed by a committee and built by craftsmen, with a perfect plastic smile and the spindles of her limbs hand-turned and gloss-varnished. When it was announced that Diana was to join the royal family, the Duke of Edinburgh is said to have given her his approval because she would ‘breed in some height’. Presumably Kate was designed to breed in some manners. She looks like a nicely brought up young lady, with ‘please’ and ‘thank you’ part of her vocabulary. But in her first official portrait by Paul Emsley, unveiled in January, her eyes are dead and she wears the strained smile of a woman who really wants to tell the painter to bugger off. One critic said perceptively that she appeared ‘weary of being looked at’....
...Kate seems to have been selected for her role of princess because she was irreproachable: as painfully thin as anyone could wish, without quirks, without oddities, without the risk of the emergence of character. She appears precision-made, machine-made, so different from Diana whose human awkwardness and emotional incontinence showed in her every gesture. Diana was capable of transforming herself from galumphing schoolgirl to ice queen, from wraith to Amazon. Kate seems capable of going from perfect bride to perfect mother, with no messy deviation. When her pregnancy became public she had been visiting her old school, and had picked up a hockey stick and run a few paces for the camera. BBC News devoted a discussion to whether a pregnant woman could safely put on a turn of speed while wearing high heels. It is sad to think that intelligent people could devote themselves to this topic with earnest furrowings of the brow, but that’s what discourse about royals comes to: a compulsion to comment, a discourse empty of content, mouthed rather than spoken. And in the same way one is compelled to look at them: to ask what they are made of, and is their substance the same as ours.'