Dropbox has nothing but a somewhat trusted brand despite repeated password hacks, and a smart filesystem driver/integration, and as a result of this a sizeable user base. Those are all that differentiate it from the army of file sharing web sites that existed prior to it. They do not have a “sync” service, they have a last-write wins distributed filesystem. There is nowhere else to go with the service as it stands.
Now Dropbox buys the iOS Mailbox app, which was previously only for Gmail. Next Yahoo announces it will be integrating Dropbox into Yahoo! Mail to make it easier to transfer large files. Will Dropbox really add their own e-mail service to the Mailbox app? Or will they cut a deal with Yahoo! Mail?