This version brings a lot of usability improvements. The WYSIWYG editor (CKEditor) has been upgraded and proposes a better dialog for the creation of links. It is also possible to configure it from the administration.
Application Within Minutes does not enforce anymore the location where the application's entries must be created, but proposes to the creator the places where and from where these entries should be. We have also get rid of the Data page where the entries where created before.
The default values of the user preferences are now better exposed to the users. The order of the applications in the application panel can now be changed by the administrator. And the users now have the ability to filter the page types when they create a page.
As usual plenty of less noticeable improvements are proposed, and 45 bugs have been fixed since XWiki 8.3.
Full release notes available at http://www.xwiki.org/xwiki/bin/view/ReleaseNotes/Data/XWiki/8.4/