Difference between Document-based and Key/Value-based databases?

Question

I know there are three different, popular types of non-sql databases.

• Key/Value: Redis, Tokyo Cabinet, Memcached
• ColumnFamily: Cassandra, HBase
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Answer 1

Ayende has given a nice explanation regarding the difference between Key-Value and Document database:

A document database is, at its core, a key/value store with one major exception. Instead of just storing any blob in it, a document db requires that the data will be store in a format that the database can understand (i.e. JSON, XML etc). In most doc dbs, that means that we can now allow queries on the document data.