If you need massive reads or writes... I'm talking hundreds of thousands of simultaneous users, then Cassandra is probably your best bet, the tooling around your use will be a bit more difficult, you may need to shard/replicate portions of your data anyway depending on use. You reach for Cassandra when your use is going to get complicated. I wouldn't look at Cassandra if I needed less than say a dozen nodes to start with.

ElasticSearch is fairly close, but I'm unsure if it will scale as well in practice. ElasticSearch handles the middle-to-high ground very well. I'm not sure I would use it for authorative data... It works incredibly well for logging (with logstash) and the front end utilities (kibana, etc) are nice too. It's primary use is as a search, if you are very read heavy for searches, or write heavy for analytical data it works well. You can tune your use to separate the storage and reads in interesting ways.

MongoDB works incredibly well when your core data is composed of mostly self-contained documents and in need of certain flexibility. A typical classifieds website is a great use case. It's also a very natural fit in a lot of programming languages, js (node) in particular is a natural fit. There's much less disconnect between the data and your application models.

RethinkDB is very similar to MongoDB, but has a more traditional mindset when it comes to it's use. I think the programming interfaces are a little better thought out, consistency and data security is at the forefront here.

In general it comes down to... very large loads, use cassandra... easiest use is mongo... rethinkdb I've been waiting on the replication story to get better, and the geosearch support is fairly recent... ElasticSearch search or for logging (write heavy). In many scenarios where I would use ElasticSearch, it would be along with an RDBMS as an authoratative data source.