I'm not knowledgeable enough to make my own argument here, but Charles Rosen argues convincingly in his book The Classical Style that Mozart, along with Haydn, basically invented the style with which they're associated. Slightly earlier composers developed the "gallant" style but Haydn and Mozart were the ones who made it a vehicle for large-scale dramatic structures based on tonal relationships and symmetrical proportions.
Mozart also introduced a degree of chromaticism to this style that was unprecedented at the time.
Mozart also introduced a degree of chromaticism to this style that was unprecedented at the time.