The most interesting advantages to me are:
- Inherent MVCC support. This means old versions of a document live after they've been updated
- Built-in searching and materialized views. I can define some metadata about my content and instantly retrieve it
Some of the competition in this case would be:
While it turns out that S3 and MongoHQ both have free offerings, the online console at cloudant is the most user-friendly (as of this second).