While SQL databases are insanely useful tools, their monopoly of ~15 years is coming to an end. And it was just time: I can't even count the things that were forced into relational databases, but never really fitted them.
But the differences between "NoSQL" databases are much bigger than it ever was between one SQL database and another. This means that it is a bigger responsibility on software architects to choose the appropriate one for a project right at the beginning.
In this light, here is a comparison of Cassandra, Mongodb, CouchDB, Redis, Riak, Membase, Neo4j and HBase: