Here are all the updates I could find people mentioning since Microsoft hasn't posted a comprehensive list of what was upgraded:
You can check out the below video for some improvements to the browser and landscape keyboard.
As I mentioned in my Zune HD browser post the mobile only support drove me nuts so I am very pleased to see the option to toggle between desktop or mobile sites. While I can now view my full ZDNet blog (not the mobile version) and go to the full YouTube site, there is still no support for Flash so you cannot watch YouTube videos on the Zune HD. I would also like to see browser history support. The browser is definitely better though and I will probably use it more for casual couch surfing than I did before.
The update may not have essential new features, but it is a worthy point upgrade and I haven't heard any reports of issues with the update.