It allows you to query json documents in a way similar to sql. redis is key/val and sits in ram, couch requires complicated design documents to query and is better as a key/val, but I'm unfamiliar with riak.

I think trello uses mongo primarily for production. technically it's feasible but I've found it to be more trouble than it's worth to scale -- too many machines are required per shard. I'm currently looking into rethink db as a replacement now though.