Neither of those things prevents something from being a database.
S3 is an excellent key/value store for large values. It's also publicly available, which is nice.
For example, all the thumbnail images on reddit are stored in S3. Essentially the client is given the key and then they can go look up the value themselves, and since it is publicly available http, it works right there in the browser.
Your example (reddit using S3) is not a database use, it is a content delivery network use. For that, it is good enough, but I'd choose a different key-value store for database, and mostly because of the latency (been there, tried that).
Alright, I can give you a better example. Netflix uses S3 to store movies that haven't been rendered yet. It's definitely a database. The server says "I need to render this movie to the iPhone format, what are the movie bits" and the "database" (S3) returns the entire movie.
Also, I would argue that static content delivery is just another form of database. It's just a massive key/value store, there the keys are the files and the values are the contents of the keys.
Let me ask you this: What is your definition of a database?
Yeah, right, the definition of the database is fuzzy. As every technology, S3 has its pros and cons, and the latency is a con this time. Take a latency-intensive application (1), and it will feel miserably slow with S3. Take a storage-intensive application with not-so-frequent access requirement (2), and it will be just OK with it.
(1) any page-load that requires 3-5 sequential DB access (based on the results that are returned in the previous steps).
(2) Netflix's "let's store somewhere until we really want to use it" qualifies.
I don't think I could come up with a coherent definition of a database which includes low-durability NoSQL key/value stores yet excludes file systems. This doesn't necessarily mean that file systems are a database, however; it could be that in order for the word "database" to be meaningful it must be defined in a way that excludes some things commonly thought of as databases, similar to why Pluto was deplaneted.
