The concept of kedusha, of holiness, appears in a vast array of instances throughout the Torah. This weeks parsha, however, is the only place in the Torah where kedusha is mentioned as an idea in itself. The Rambam treats this statement as a general warning to keep the commandments, and not as a separate mitzva. The Sifrei similarly explains that kedusha refers to the holiness of the commandments. Rashi goes on to say that within the laws protecting one from forbidden relationships, one finds kedusha.
Ramban views kedoshim tihiyu much differently.
He does not restrict the idea of kedusha to specific laws. Nor does he consider kedoshim a general overview of the importance of the mitzvot. Rather, Ramban treats kedoshim tihiyu as a mitzvah in and of itself. He explains that the pasuk, in a sense, covers all of the issues that are naval birshut haTorah - issues that technically fall within the legal boundaries of the Torah but are not in the proper Torah spirit. For instance, as long as a person is keeping Kosher, he might think that he can indulge in gluttony.