One thing Dropbox does which I think is pretty easy and yet noone has done is their Apps.

As a developer you can create a dropbox App which means there is a specific subdirectory in the users dropbox folder which you have access to. That's it, really simple. This lets me create for instance a purchase order distribution system in which the company drops their purchase orders in a folder and they will be distributed to their vendors with email notifications. What other system facilitates this?

In contrast Google Drive apps are all or nothing. You cannot ask for permission on a single folder!

Are there any other services that can do this like Box.net?