WSD is does not mean step-through. Step-through is simply a type of frame, commonly called a mixte or the woman's version of a hybrid. Waterford, Bob Jackon, Mercian, etc. are a few of the very short list of companies that I know of that actually make serious road bikes in a step-through or mixte design; you probably won't run into any of these unless you try really, really hard. Many standard road bikes today have sloping top tubes but not enough to be called a mixte.

Shorter top tube, maybe shorter cranks, narrower bars, shorter reach brakes, maybe lighter tubing, different saddle. There are a number of things that may make a bike "WSD." Of course, some companies simply paint it a different color and call it WSD.