A few highlights of the new version:

* XWiki now requires Java 6!
* Upgraded versions of libraries which also no longer support java 1.5.
* Code removal and re-factoring to strengthen the platform foundation.

You can review the changes made and bugs fixed since 3.0 by going to:
http://www.xwiki.org/xwiki/bin/view/ReleaseNotes/ReleaseNotesXWikiEnterprise31M2

And download the new version:
http://www.xwiki.org/xwiki/bin/view/Main/Download