Even though Beethoven is included with Haydn and Mozart as a classical composer, how does his style differ from theirs?
Although Beethoven's music is considered, for the most part, in the classical tradition of Haydn and Mozart, his style differs from classical in many ways, especially in his later periods. In his earlier periods, his music was more like the other classical composers, but he evolved and is considered a bridge between the classical and romantic periods.
He used larger orchestras and wrote music that featured the violas and cellos, not only the violins, so his music sounded richer. He also used notes and melodies in a different, unexpected way, making his works seem very much a departure from the strict classicists. Listening to his symphonies, you can really hear the grandeur in the composition, a style that was copied by Wagner and other artists of the Romantic period. His works are also very evocative and emotional, not as "neat" as classical music, much more "over the top". These are not musical terms, to say the least, but I am a fan of Beethoven and this is how I describe listening to his music - it's just so LARGE compared to Mozart and Haydn. Not something you could dance a minuet to, that's for sure.
This is very basic information that I remember from studying music. Perhaps a music expert can give you more specifics regarding musicality. There is much good information available if you do some digging on your own. See one good link below.
The Romantic tendencies seems to differentiate Beethoven from his Neoclassical counterparts. In my mind, Beethoven's vision of himself as the composer allowed his work to transcend the traditional conception of the role under Neoclasicist parameters. In the time of Haydn and Mozart, the composer was supposed to be secondary to the music. In Beethoven's case, he was the music. Beethoven understood that the music and the composer are one in the same. He possessed an understanding of fame and how to manipulate it as a composer in order to generate a sense of excitement about his work. I think that Beethoven also understood the political dimension that the artist can hold in reference to both his music and his world. He understood that Beethoven could hold political beliefs and allow his music to represent such a reality, a conception that was not necessarily embraced in the Neoclassicist style. Both the vision of the artist and the notion of social and political reality in one's work represented Beethoven's departure from Neoclassist principles.