Road bike shoes should be as stiff, light and aerodynamic as possible. In combination with clipless pedals, they're an extremely effective combination. Road bike shoes use larger cleats than MTB 'SPD' clipless pedals, and unlike mountain bike shoes, how well you can walk in them plays virtually no role at all. A good fit and precise adjustability via rotatable dials, Velcro straps or ratchets is very important. To reduce weight and maximise stiffness, high-end road bike shoes often have carbon soles; after all, weight savings are particularly noticeable when rotating.