I'm not an expert on couchbase (and neither on RethinkDB, to be frank, though I am a huge fan), but here's what RethinkDB has going for it:
- Changefeeds - easily open a persistent connection to the server and get updates when the results of an almost arbitrary query changes.
- Joins
- Expressive query language that is pretty functionally minded, really shines in their clojure/haskell drivers
- Excellent client libraries, well maintained
- Geospatial queries/objects
- Amazing admin interface (it has been amazing for a long time, too, not a recent change)
- First class consideration of replication & sharding (it is not a bolt-on in any way shape or form)
- API-driven cluster configuration
- API driven permissions management (this is relatively new)
- Excellent, easy to follow documentation
There are more things, but this is just what I can think of off the top of my head.
The team at RethinkDB is also just great -- I've met them in person and gotten help from them and they're straight shooters.
They've also got this great project coming up called Horizon: https://www.youtube.com/watch?v=Sb1lH5mvYmU
Also they have a video up with a member of the team building a realtime game with React Native: https://www.youtube.com/watch?v=xRK0SYSgVF0
Maybe someone who is very familiar with couchbase can help make a list... I'll start it off:
- Custom query language
- First class consideration for scale -- replication and sharding