It's weird that things as different as S3 and Redis all fall under the umbrella of "NoSQL". We should probably replace the SQL-NoSQL dichotomy with terms that more accurately reflect the real differences.
Since when is S3 a 'database'?
It's got an API, you can store data in it and retreive it later by key, since when was S3 not a database? It's a key/value store that lets you store very large values.
Not really a "distinctive" one (a la mongo, couch, etc.), but I see your point.