First of all, think that they got catwalk-inspired looks (think Topshop, to name just one) on a budget and that means a lot.
They are not afraid to mix and match pieces that just don't seem to be made to go together.
They focus on their individuality, not paying too much attention at looking fashionable but more at being themselves.
Look at this picture of Alexa Chung: she's adorable! Love the colors she wears (nude/blush has always being my fave), the Luella bag and the H&M striped cardigan say she surely got sense of humour and doesn't take herself too seriously. I like that she plays with who she is.