Earlier quoted context omitted.
What's the difference between Cayley and Neo4J?
Think about Datomic. It can use Riak as a backend or something else but it is still seen ad Datomic to the outside world. (You can also compare it to Lucene vs Solr)
Wait, Datomic with Riak as the backend? That seems a ridiculous level of abstraction, since Riak itself uses pluggable backend stores (Bitcask or LevelDB). Like creating a programming language whose interpreter runs on the JVM.