on is mostly about stabilization before 7.0 final. It contains diff UI improvements, Extension Repository improvements, a new component based watchlist module, a new application to edit wiki skins. XWiki also move to Servlet 3.0.1 which means various old application server versions are not supported anymore.
Full release notes available at http://www.xwiki.org/xwiki/bin/view/ReleaseNotes/ReleaseNotesXWiki70RC1
See http://jira.xwiki.org/secure/IssueNavigator.jspa?reset=true&jqlQuery=category+in+%28%22Top+Level+Projects%22%29+and+fixVersion+in+%28%22%37.%30-rc-%31%22%29+and+resolution+in+%28%22Fixed%22%29 for more details