Currently (as of WordPress 3.8), plug-ins have a changelog, so that you can see what has been changed before you install the update.
Themes currently do not have changelogs. The "updates required" page in the dashboard simply says that there's an update to the theme, and then gives you the link to update it. If I have a child theme, how do I know if changes in the parent theme (which has been updated by the developer) will affect my child theme?
I think it would be a good idea if themes implemented the changelog concept, just like plug-ins.