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It would be really helpful to be able to search the comments made on the files.
I would also appreciate ability to search through tags and comments on both files and folders.
Without that feature, DropBox can't really be called an Advanced storage solution (simple syncing between PC and file server can be done with a cheap FTP server).
Currently, users have to use external application for searching such as http://cloudfindhq.com/ or Evernote, but that over-complicates searching and finding files.