- Unconventional structure: The symphony comprised five distinct movements, departing from the traditional four-movement structure of symphonies at the time, which was considered innovative and avant-garde.
- Programmatic nature: The symphony was inspired by personal experiences, emotions, and a story about an artist's obsessive love and subsequent torment, a concept known as "program music" that wasn't widely prevalent in symphonic works.
- Bold orchestration: Berlioz employed an expanded orchestra with unusual combinations of instruments, including the cornet, ophicleide, and harp, creating a rich, colorful, and expressive sound that was both captivating and unfamiliar to audiences accustomed to more conventional orchestral arrangements.
- Use of leitmotifs: Berlioz introduced the idea of leitmotifs, where specific melodies or themes are associated with particular characters, emotions, or ideas throughout the work. This technique was novel in symphonic music at the time, adding depth and emotional resonance to the composition.
- Musical depiction of extrasensory experiences: The symphony's final movement, "Dream of a Witches' Sabbath", featured a musical representation of the protagonist's nightmare and the macabre atmosphere of a witches' gathering, complete with uncanny melodic, harmonic, and rhythmic elements. This depiction of otherworldly experiences was groundbreaking and unsettling for contemporary listeners.
Overall, the combination of these elements—programmatic narrative, unconventional structure, innovative orchestration, leitmotifs, and unusual musical depictions—made the Fantastic Symphony a radical departure from traditional symphonic music and startled Parisian audiences who were accustomed to more conservative works.