This version opens the new 9.x cycle. It contains all improvements introduced in versions 8.4.1 to 8.4.4, and brings admin and developer features mostly. Among other things, XWiki deals better with big attachments now.
Full release notes available at http://www.xwiki.org/xwiki/bin/view/ReleaseNotes/Data/XWiki/9.0RC1
See http://jira.xwiki.org/secure/IssueNavigator.jspa?reset=true&jqlQuery=category+in+%28%22Top+Level+Projects%22%29+and+fixVersion+in+%28%22%39.%30-rc-%31%22%29+and+resolution+in+%28%22Fixed%22%29 for more details