The magic of Dropbox is that it strikes just the right balance between simplicity and power.
When my mother deleted a book that she had been writing for almost a year (and its backup), I was able to show her how to retrieve deleted documents from Dropbox. It was incredible.
Arthur C. Clarke once said that any sufficiently advanced technology is indistinguishable from magic. Well Dropbox is magic to most of the people that I have introduced it to.
I know that there are other solutions out there that do similar things to what they do, but I have never really been able to find one that just clicks with so many people regardless of technical ability.