RethinkDB has been really mindful of its' consistency, has an in the box story for observable collections, and a really great UI out of the box. I really hope I get to use it for more than toying around with.

I'm not sure that I'd choose Mongo over alternatives these days, if I'm on AWS and can use RDS, I'd go PostgreSQL mostly, but RethinkDB, ElasticSearch, C*, and others also have their places.