With the last beta version, build 13C59, Apple once again asks developers to focus on Mail, Messages and Graphics Drivers, according to people who have seen the most recent release.
Continuing from the last build, focus areas include SMB2, Mail, Messages, Graphics Drivers, VoiceOver and VPN. Mavericks' messaging functions, especially Mail, have been the source of trouble since the OS first launched in October, as affected users saw Gmail compatibility issues and erratic Smart Mailbox behavior. A subsequent standalone patch was released, and the problem was supposedly addressed in the OS X 10.9.1 update, but complications still exist.
Developers can download the latest version of the upcoming Mavericks maintenance update via Software Update or Apple's developer portal.