Being able to preview Office files within Confluence has been a wonderful improvement. I'd like to see it extend further and allow .eml and .msg (saved email) files to be inline viewed when attached.
Being able to do this would further the cause of weaning our organisation off email, by transferring discussion into Confluence where it belongs. We have users attaching email files to Confluence pages already, but at present this tends to wall off the information content in the email unless someone goes to the trouble of opening the file.