We sometimes need your permission to do what you ask us to do with your stuff (for example, hosting, making public, or sharing your files). By submitting your stuff to the Services, you grant us (and those we work with to provide the Services) worldwide, non-exclusive, royalty-free, sublicenseable rights to use, copy, distribute, prepare derivative works (such as translations or format conversions) of, perform, or publicly display that stuff to the extent we think it necessary for the Service. You must ensure you have the rights you need to grant us that permission.
On first pass, that sounds rather scary. If you post your soon-to-be-outrageously-successful novel on Dropbox, are you giving Dropbox permission publish it and even create translations or a movie based on it?
As a number of Slashdot posters have pointed out, however, that bit of legal boilerplate is common among the providers of online services. In our lawyer-afflicted society, it prevents nuisance lawsuits. Dropbox's ability to copy etc. is limited by the opening and closing terms:
We sometimes need your permission to do what you ask us to do with your stuff (for example, hosting, making public, or sharing your files).
... to the extent we think it necessary for the Service. You must ensure you have the rights you need to grant us that permission.
Since Dropbox is not in the book or movie-making business, publishing or making movies out of what we've written is not doing "what you ask us to do with your stuff" nor is it "necessary for the Service."
That said, those terms do point out that, at a practical level, we're still responsible for the legality and security of what we post online. As the last sentence in that paragraph points out, don't post to Slashdot material whose copyright you don't own or that isn't covered by fair use.
Also, especially given Dropbox's recent blunder in which it left its system totally unprotected by any passwords for several hours, it might be a good idea to encrypt the more critical material you post online. I've not tried them yet, but there are apparently ways to encrypt Dropbox folders. Doing that might be especially appropriate if you're using Dropbox as an off-site backup for an entire Scrivener book.
If Dropbox's security fails again and bad guys try to take advantage of that, even a modest amount of protection will probably mean that they go after easier game and leave your "stuff" alone.
--Michael W. Perry, Seattle