[moe]

redis.io has simple pages describing every command in the system and, critically, the associated algorithmic complexity and discussion of likely issues and problems.

This. A thousand times.

Every database should be required by law to have that kind of reference. With most of the NoSQL databases it's ridiculously difficult to figure out basic things like "does this support range queries at all?" or (god forbid!) "how would I implement range queries efficiently?". You can be lucky when they casually mention which concepts their DB was originally based on (bigtable somethingsomething dynamo something) to get at least a rough idea of what to expect.

Writing documentation is hard. But many of the contenders (including riak) are so overtly hostile that it seems almost deliberate.