RethinkDB is in my mind firstly Mongo done right... Cassandra is a very different beast, but seems to be the better option when you need massive (100+ node clusters) scaling, but that comes with a lot of work in terms of development.
While RethinkDB is probably most comparable to Mongo, it's worth noting that the sharding/replication/fail-over support and model is much better than Mongo's. Beyond that, the update notifications (streams) are in the box, where with mongo it's bolt-on. Also, rethink supports joins at the server (though best to avoid a lot of the time).
The admin ux is pretty awesome, and the dev team has been very cool to follow.