AFAIK, right now, there's no convenient S3 desktop client that makes it trivial to automatically mirror and share folders, much less make client-side encryption

AFAIK, right now, there's no convenient S3 desktop client that makes it trivial to automatically mirror and share folders, much less make client-side encryption of files trivial on top of all of the above. IMHO, that's a very viable niche... User experience matters – it's not solely about cost. Also, about n-crypted cloud – last time I looked, the file size limit was 150 MB – that kills the utility for my team, but it's still a decent option from my experience...