Cassandra isn't easy to learn like, say, couchdb. But Couch uses JSON (An awesome choice, BTW), and Cassandra uses Thrift. Cassandra is kinda difficult to pick

Cassandra isn't easy to learn like, say, couchdb. But Couch uses JSON (An awesome choice, BTW), and Cassandra uses Thrift. Cassandra is kinda difficult to pick up because there is no SQL equivalent, there are no relationships, joins or "where"s. So, basically, it's an engine without user friendly controls. But - it's probably the most awesomely powerful storage engine yet available in the public domain. Imagine if Google released a server image of one of their storage nodes... ostensibly, that's what Facebook did with Cassandra.