> Adding a sharing layer on top is something easily replicable.
Considering that Dropbox is the one company that has just "made it work", I wouldn't be so sure of that. Syncing isn't always easy and at scale the problems are that much bigger. Mix in cross platform issues (not just Mac/Windows, but mobile too) and it gets that much more interesting.
There's a subtle difference between "sharing" (which I was talking about) and "syncing." The examples in the previous comment focused on sharing (music sharing, photo sharing) and thus were really about the magic of cloud storage like S3, not anything unique added by Dropbox.
You're absolutely right that syncing is a classically hard problem that Dropbox has addressed impressively well for consumers.
Considering that Dropbox is the one company that has just "made it work", I wouldn't be so sure of that. Syncing isn't always easy and at scale the problems are that much bigger. Mix in cross platform issues (not just Mac/Windows, but mobile too) and it gets that much more interesting.