The highlights of this release are the new page type filter from the Create Page form and the new configuration section for the CKEditor. Changing the supported languages from the administration has been made easier with the new language picker. Ordering the Application panel items is now possible with simple drag & drop. Sub-wiki initialization is now asynchronous. As usual the release also includes many bug fixes and smaller improvements.
Full release notes available at http://www.xwiki.org/xwiki/bin/view/ReleaseNotes/Data/XWiki/8.4RC1/
See http://jira.xwiki.org/secure/IssueNavigator.jspa?reset=true&jqlQuery=category+in+%28%22Top+Level+Projects%22%29+and+fixVersion+in+%28%22%38.%34-rc-%31%22%29+and+resolution+in+%28%22Fixed%22%29 for more details