This release comes with Solr search enabled by default. The search UI has been redesigned and the search backend has been greatly improved. A new Menu application is now avaible to help you create navigation menus that ca be placed after the header or in a side panel. Beside this, a lot of bug fixes (124) and small improvements (53) make this release worth trying.
Full release notes available at http://www.xwiki.org/xwiki/bin/view/ReleaseNotes/ReleaseNotesXWiki51
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.%31%22%29+and+resolution+in+%28%22Fixed%22%29 for more details