The same clothes I'd wear if I'd walk to work, i.e. regular clothes, army style boots, skiing jacket. Below -10C I'd wear thicker (ski) gloves. -15C is my lower limit for riding - mainly because I don't have any special winter attire (face mask being the main needed thing), which in turn stems from the fact that temps below -15C have been very rare here in recent years.
That said, my commute isn't very long - 9.5km one way which takes roughly 30 mins or slightly longer in bad weather, so I don't really start to freeze yet.