This is the first of the 2 stabilization releases that happen at the end of each yearly Cycles. Lots of polishing has been done, especially for the recently introduced Nested Pages feature and its consequences on the UI redesign (modified menus for example). Some extensions started exploit nested spaces to bring some improvements.
Full release notes available at http://www.xwiki.org/xwiki/bin/view/ReleaseNotes/ReleaseNotesXWiki73M2
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.%33-milestone-%32%22%29+and+resolution+in+%28%22Fixed%22%29 for more details