Developers have been advised to focus their testing on Mail, Messages, VPN, graphics drivers and VoiceOver while using the beta version of the update, which comes in at build number 13C32.
The key new feature in the upcoming Mavericks update is FaceTime audio calls, Apple's VoIP solution that is currently only available on iOS devices. In OS X 10.9.2, Apple has added FaceTime audio to both the FaceTime app and the Messages app. During a FaceTime audio call, Mac users can also switch between video and audio mode.
Apple's introduction of a native VoIP solution could prove to be a blow for third-party services such as Skype, though Skype does allow calls between Macs and PCs, whereas FaceTime will presumably only work between Apple devices.
It's currently unknown when Apple will release the OS X 10.9.2 update to the public, but we'll update this article as soon as we find out more. OS X 10.9.1 was first seeded to developers more than a month before it was released to all Mac users, though.
See also: How to make phone calls on your Mac