The more I think about it, the more I like Kate's minimalistic style. For the most part, I think she dresses in what could be termed as "serviceable" attire. Its appropriate, it doesn't scream out anything at you like "look at how flattering and trendy I am" and with her hair always being basically the same, there's no jumping on a bandwagon about different styles.
I believe its all done on purpose. Just another dress where they can name the designer, the color and the cut and nothing more be said. It also allows more focus to be on what Kate is doing rather than what Kate is wearing.
Smart woman.