Mike @ RethinkDB here-- the document model is similar to MongoDB in that you store JSON documents, and there are no schemas, so it's not a relational database.
Our query language (ReQL) does support a lot of SQL-like queries, such as group-by and JOINs, but there are no relations defined.
It's also worth noting that a great team has sprung up to build a C# / .NET driver for RethinkDB (https://github.com/mfenniak/rethinkdb-net).