Jim Webber (Neo4j):
In these kind of situations, choosing a non-graph store for storing graphs is a gamble. You may find that you’ve designed your graph topology far too early in the system lifecycle and lose the ability to evolve the structure and perform business intelligence on your data. That’s why Neo4j is cool - it keeps graph and application concerns separate, and allows you to defer data modelling decisions to more responsible points throughout the lifetime of your application.
You can store single-step acyclic relationship in every kind of storage, but that doesn’t make it a graph. Nor a graph database.
Original title and link: Graph Database Advantages over Document Databases (NoSQL databases © myNoSQL)