Dropbox is wonderful for storing and syncing files, but it ignores a key part of personal and business storage: e-mail. Many if not most e-mail services provide limited space for e-mail messages. As a result, users are forced to delete previous e-mail messages to make room for new ones.
Dropbox could easily solve this problem by adding support for the IMAP protocol. Dropbox users could then configure their mail clients to use Dropbox as a mailstore. This would allow e-mail messages to be moved to Dropbox for storage and retrieval from any IMAP-compatible mail client.