The libexo framework for preferred applications is great and allows the user to normalize and synchronize their XFCE experience. Right now we have "TerminalEmulator", "WebBrowser" and "MailReader".

It would further help to create a solid experience to be able to choose also other kinds of "expected" preferred applications. I'm thinking of the kind that covers functionality by mainly addresing a "task" just like the previous ones had. What about, for example:
 "FileBrowser" (one could pick between Thunar, XNC, Konqueror, Rox, ...)
 "FileArchiver" (one could pick between Squeeze, XArchiver, FileRoller, ...)

Those are the ones that ring my bell: applications that introduce users to their system, their task is as simple as the word, and are of particulary common usage. It wouldalso allow to preserve familiarity with common tasks for people migrating from other DE's.