Stage Dressing is the Key
It is perhaps easier to think of steampunk as an aesthetic rather than as a genre, this is because virtually any genre of literature, that does not have a specific place in time, can be set down in a Steampunk world and still be true to itself as well as its new setting. Imagine, for instance, a reclusive old gentleman, sitting back in his musty old armchair. He has just handed down a family heirloom to the son of an old comrade. The boy is fascinated by the light catching the razor-sharp edge of his father's sword as he tests its balance. The old gentleman nods his head in appreciation and says, "Not as clumsy or as random as a revolver; an elegant weapon of a more civilised age." Obviously this is a scene from Star Wars paraphrased, but if you imagine this scene in a Victorian gentleman's study, it works beautifully without any drastic alterations. With only a little thought on the matter we can see that Steampunk has a flexibility that we just don't see in any other accepted genre. War, romance, fantasy and even gritty detective stories, all will fit beautifully in a Steampunk world. Many types of story seem at home, quite naturally, in the setting; Shakespeare's Macbeth or Richard III would be amazing in a Steampunk setting.