This article by Wong and Proctor summarizes the composition nicely:
St. John Passion
Bach's St. John Passion is gloomy, stressful, highly emotional, and powerfully meditative. Its depth comes from its subtlety. There is no noble hero, no mustache-twirling villian, no hummable tunes. As difficult as it was to work within the confines of John's text, Bach was able to create a moving work with musical, spiritual, and psychological unity of form.