Here is a very short summary.

RethinkDB is much easier to set up and operate, has a much more powerful query language, and is much friendlier for application developers.

Cassandra supports high write availability in case of network partitions. RethinkDB does not. The flipside of that is that you (as an app developer) have to deal with conflicts in Cassandra which makes writing applications much more difficult.

If you need high write availability in case of network partitioning, go with Cassandra or Riak. If you don't, go with RethinkDB or MongoDB.