Shakespeare structured his plays to be layered and complex. They could have deep meanings lying in philosophy, ethics and psychology. They could also be enjoyed by those whose sole purpose was to watch them as lighthearted entertainment, as most of the Elizabethans did.
While the structure of his plots may not have been original, his plots are far from repetitive, even though it may have seemed so to certain members of the audience. He took creative liberties with pre-existing plots and transformed them into something that was exclusively his, bejeweled with elements which were outrageous and bizarre; be it the Moor and his fair Desdemona, or the shipwrecked story of the twins in Twelfth Night. His plots were intricately spun with parallels, reversed symmetries and repetitions, enough to make them analogous and distinct at the same time.
As for the characters being on extreme ends of the spectrum, Shakespeare molded them so as to make them contrived with a glint of realism. This worked in several ways. It helped a larger part of the audience to identify with the different idiosyncrasies of the characters and helped them envision the consequences their vices may have. It was also a way for Shakespeare to make socio-political statements without making it apparent.