And also to share files with other people, or access them via the website.

Wuala can deduplicate encrypted data. The encryption is a little bit weaker than standard encryption (because you can tell if two users are hosting the same file), but it's not possible to determine a file's content from the cipher text (if the file is unique).

Good opportunity for a paid level of service. Client-side encryption for $X extra per year, to make up for the extra storage used.

Explain that to users AND businesses who trust Dropbox with sensitive information.