"Being document-based datastores, Riak and CouchDB are the most direct competitors to MongoDB" But Riak is a Key-Value store, not a document one. If that's the premise I wonder how illuminating the rest can be (I kept reading: it's not)
Riak has secondary indices, and map/reduce for ad-hoc queries. You can store raw binary data in it, and it's happy with that, but if you store JSON then it can query it. From what I can tell, the biggest difference between Riak and Bigcouch from a data model POV is that Bigcouch has materialized views, while Riak's are ad-hoc. I'm not an expert in either though...
"For better or worse, many people consider MongoDB and Riak to be competitors. In reality, there are very few similarities between the products."
http://seancribbs.com/tech/2011/11/07/mongodb-and-riak-in-co...