At a glance, UI. Dropbox is horrible for restoring things - it's possible, but it's definitely not the primary use-case. Especially with something visual, you can go a lot further a lot faster if you can see what it was at each state without having to hit the back button a bazillion times.
What about layering designer-specific functionality on top of dropbox? Use the Dropbox API, people sign up and tell you where in their Dropbox folder they store designs, and you provide specific functionality, like the ability to compare versions of a design in an intuitive way.