Basically, the colour.
A man and women of similar proportions will need a bike of similar dimensions. Having said that, there are enough general trends in the relative sizes of various parts of a man and women for manufactures to claim that they can make frames that beter suit one or the other.
But a decent bike fitting service will fit you to whatever frame is appropriate. Mrs TB fits regular frames okay. But Paul Hewit has made 24 inch wheeled frames for some women if that is what their sizing requires. But a bloke of the same size and proportions would also need one of these!
However Mrs TB does prefer the Deda short reach 'female' bars, but hates the patronising, cynical 'women specific' marketing blurb that most manufactures come out with.
Again saddles are often marketed as being male and female, but members of each sex have arses of such wildly different sizes that restricting your choice to what some marketing department has decided is 'for you', might exclude your perfect seat from your search.