Mac OS X is on a yearly update cycle, so it isn’t surprising to see that Apple is already actively testing early builds of the next OS on internal machines. OS X 10.9 will likely be another fairly limited upgrade, as Apple is pushing features on yearly updates instead of holding back and releasing a deluge of enhancements every few years.
OS X 10.9 will also likely improve battery life and performance throughout the OS, given Apple’s focus on smaller, lighter, and more portable machines such as the MacBook Air and new retina MacBook Pro line. It’s also possible that OS X 10.9 will see some pretty major UI and UX changes, given Johnny Ive’s new control over all of design and user interaction at Apple. So, in regards to looks, it could likely be a very major update.