All of the benefits sound the same as MongoDB, which also does sharding and JavaScript queries, including arbitrary code in "callbacks" in map-reduce or $where clauses (though it's slow).
Can someone explain the benefit of RethinkDB over MongoDB?
RethinkDB is based on a fundamentally different architecture from MongoDB. Instead of polling for changes, the developer can tell RethinkDB to continuously push updated query results in realtime -- check out http://rethinkdb.com/faq/ for more details on this.
Just FYI, Rethink does sharding and JavaScript queries (including callbacks) -- http://rethinkdb.com/api/javascript/js/. The `js` command interops with every other command in RethinkDB and works really well.
Can someone explain the benefit of RethinkDB over MongoDB?