What's interesting about key-value databases? Storing keys and values in an RDBMS is trivial... Graph databases are great for datasets where structure matters

What's interesting about key-value databases? Storing keys and values in an RDBMS is trivial... Graph databases are great for datasets where structure matters more than in a relational database and (way) more than in a K/V-store. When you're doing traversals of arbitrary depth, other technologies fall on their collective face.