I've just been reading a fashion historian talking about the lack of coronation tiaras, and what a major change it is for the royal family. He thinks it signals an attempt to be more relevant to the population, especially at a time of economic troubles. Also, as only Charles and Camilla wore crowns, it effectively enhanced their status within the family group.
To me, those two ideas seem contradictory, as by exaggerating their status, Charles & Camilla become more distant, and so less relevant.
While I rather liked Kate's headpiece, I don't really see the difference between her wearing that, and wearing an actual tiara. Obviously, it was cheaper to create, in fact it looked simple to make, something say a creative bride might make for herself. But she already had tiaras available at no cost, so why not use them?
I especially thought it was a pity that Camilla's two ladies in waiting (the emergency Camillas) didn't wear some sort of headdress, they looked quite plain in their white dresses. Had the royal women worn tiaras, the LIW could have worn simple coronets or wreaths.