Final answer:
The Eleusinian Mysteries were unique due to their exclusivity and lack of religious significance. They were open to people of all social classes and ages and focused on personal transformation and initiation.
Step-by-step explanation:
The Eleusinian Mysteries were nearly unique among ancient Greek rituals due to their exclusivity and lack of religious significance. Unlike most Greek rituals that were accessible to only certain groups, the Eleusinian Mysteries were open to people of all social classes and ages. Additionally, while other rituals focused on religious worship, the Eleusinian Mysteries were primarily focused on personal transformation and initiation.