A new Android changelog has been posted and it rolls out as we speak to all Nexus devices. Android 4.4.4 (KTU84P) is mostly a security update and it basically includes Android Open Source Project changes.
The update does not include any significant proprietary component from Google changes or any manufacturer updates. One of the most important changes is the fixing of Open SSL vulnerability. This update changes a few things in Open SSL, WebView and Chromium projects. Some logging and security bugs were also fixed.
Android 4.4.4 proves to be a minor update that will make Android devices less vulnerable. Funky Android has more information regarding the update.