Any other content in the data directory (sessions, quizzes, local plugins...) should be moved elsewhere. I would propose the core directory again however I don't know why quizzes would be written to disk in the first place. Sessions is not too big a deal mostly as I figure if someone is using filesystem based sessions, then they are only on a single server anyway. This should be the last thing required to make Moodle's file API more friendly to multi-server implementations.