The 6.4 release is the last major release of the 6.x development Cycle and as such was mainly dedicated to improvements and bugfixes on things introduced during 6.X.
This version introduces some developer oriented improvements: allow wiki based skins to overwrite macro.vm template, allow filesystem skins indicating explicitly the skin they are inheriting from, allow any component to be injected with its ComponentDescriptor, Panels and UI extensions are now executed with the right of their author so among other things they can use APIs requiring Programming rights.
It also brings UI improvements in the Menu application, a new Mail application (and Mail-related improvements, such as the ability to send massive number of emails) and lots of Flamingo skin improvements (Mobile-friendliness to mention one), while offering developers the ability to write LESS in Skin Extensions, and a cool icon picker.
Last but not least, this version provides the ability to rate Extensions inside the Extension Repository and visualise them inside the Extension Manager.
Full release notes available at http://www.xwiki.org/xwiki/bin/view/ReleaseNotes/ReleaseNotesXWiki64
See http://jira.xwiki.org/secure/IssueNavigator.jspa?reset=true&jqlQuery=category+in+%28%22Top+Level+Projects%22%29+and+fixVersion+in+%28%22%36.%34%22%29+and+resolution+in+%28%22Fixed%22%29 for more details