WordPress have listed the bugs that are fixed as,
Fix some issues with older browsers in the administration area.
Fix an issue where a theme may not preview correctly, or its screenshot may not be displayed.
Improve plugin compatibility with the visual editor.
Address pagination problems with some category permalink structures.
Avoid errors with both oEmbed providers and trackbacks.
Prevent improperly sized header images from being uploaded.
The security issues that are fixed included potential privilege escalation and a bug that affects multisite installs with untrusted users.
Do not forget to backup your files before you update. My clients please do not worry, we will do the upgrades for you.