This is mostly a stabilization release leading to XWiki 5.2 final and it brings a new "Home" top level menu (available when there is more than one wiki) and a new Distribution Wizard step to upgrade all wikis. The WikiStream framework has been improved and now can be used to export large wikis in XAR format. Besides this the new release includes 49 bug fixes and 16 small improvements.
Full release notes available at http://www.xwiki.org/xwiki/bin/view/ReleaseNotes/ReleaseNotesXWiki52RC1
See http://jira.xwiki.org/secure/IssueNavigator.jspa?reset=true&jqlQuery=category+in+%28%22Top+Level+Projects%22%29+and+fixVersion+in+%28%22%35.%32-rc-%31%22%29+and+resolution+in+%28%22Fixed%22%29 for more details