To clarify what exactly makes classical music, well, classical music, and what makes it so great and full of rich depth we will take an adventure into an element of it: Counterpoint. Counterpoint is the relationship between varying voices, rhythms, and harmonic elements. Written typically with 2-4 voices, the elements of counterpoint all work together and in contrast with each other to make brilliant harmonies and consonances to bring a piece to a more whole, full piece. This is also a big element in music theory which is based upon the 'rules' that Johann Sebastian Bach paved for future composers. I implore you to check out this link on Counterpoint via Wikipedia for a more in depth explanation.