First what I like best is the transparency of the RethinkDB team and its dedication to make RethinkDB a user-friendly product.
The website is full of information about the current bugs and performance issues. And there is a real effort to make the product very user-friendly: a nice documentation, a beautiful UI, a clean and well-thought API, simple sharding and replication features, joins, real-time features...
It really has lots of attractive features for such a young product.
RethinkDB is still quite slow for many operations. I know a Swiss start-up that switched from RethinkDB to PostgreSQL because of performance issues. These problems could be fixed in the future but if you need good performance right now at the very least do some benchmark and load-testing before investing too much in this database.
- It makes unit tests more difficult to do. A database with a large API containing plenty of functions is harder to mock than a SQL database with a leaner API.