1. As a young man in Paris, Berlioz became fascinated with opera. Here was a genre that combined all the arts--literature, drama, music, poetry, architecture, and so on. It was probably this theatrical bent that led him to explore similar dramatic representation in his instrumental works.
2. The Symphonie fantastique is an important work not because of the program, but because of how Berlioz handled the musical elements. The power of the melody and his novel treatment of it as a unifying device, the formal design, his use of harmony, and especially his orchestration were simply fantastic.
3. One of the problems Berlioz encountered in getting his works produced was the cost involved. His orchestrations were monumental, requiring incredibly large groups. You might want to obtain a score of the symphony and compare Berlioz’s orchestra with that of Beethoven and Mozart.