So, as per my normal proclivities, I have read all available documentation and hereby declare myself an expert on the subject.
Looking at the press on S3, particularly looking at the smugmug success story, I'd got the impression that S3 was more than a large pay-per-use filesystem, which it's not. It is, however, a very, very well thought-out large-scale decentralized high-availability filesystem, which is not to be sniffed at.
However, I don't understand why SSL is (or is intended to be) mandatory for SOAP requests, whereas it's not (as far as I can tell) for the REST API. I'm sure there's logic to the design choice, but anyway.
My other comment is that, given the ability to can attach metadata to objects, it's kind've a shame you can't query on that metadata, even at a relatively simple level. This would be great for e.g. the purposes of tagging, although sundry other use cases present themselves, if you think of the potential of storing stuctured, as opposed to opaque, data.
Despite the above gruntles, Amazon are still closer than any other player to reaching a true commoditization of the basic computing resources: CPU time, storage space, and bandwidth. As Bezos says, there's an awful lot of friction involved in setting up a web service. Anything that can reduce that interests me.