All Jewish holidays represent different aspects of the Jewish people's relationship with G-d while Shavuos represents the relationship itself.
Why is Shavuos also called "Atzeres" which means "holding back"? Why are there no special mitzvos associated with Shavuos? Why are Pesach and Sukkos seven days while Shavuos is only one day? These questions are answered by understanding Shavuos as a "blank check" between the Jewish people and G-d.