What were Nagarjuna's seminal contributions to Buddhist philosophy?

Nagarjuna's ideas are considered to represent the historical starting point of Mahayana Buddhist philosophy. Can his ideas be considered a clarification of the philosophy presented in the Pali canon? And if so, what are the specific ideas in Nagarjuna's thought that are important for that clarification? Or, am I completely wrong in my assumption that Nagarjuna actually contributed anything significant to Buddhist thought? (I'm not afraid of criticisms of my pronouncements).