This release brings many improvements to the Mail Application and its API, including support for mail throttling. A new Wikis category is available in the administration that groups together sections with wiki-related configuration options. Other smaller improvements (23) and a few bug fixes (15) complete this release.
Full release notes available at http://www.xwiki.org/xwiki/bin/view/ReleaseNotes/ReleaseNotesXWiki64RC1
See http://jira.xwiki.org/issues/?jql=category%20in%20%28%22Top%20Level%20Projects%22%29%20and%20fixVersion%20in%20%28%226.4-rc-1%22%29%20and%20resolution%20in%20%28%22Fixed%22%29 for more details.