This update fixes an issue that could prevent L2TP VPN connections from working with OS X Server, as described in http://support.apple.com/kb/TS5313The issue has been a major problem for a number of users, as reflected by over two-thirds of reviewers giving the 3.0.1 version of OS X Server a one-star rating in the Mac App Store. [Direct Link]
This update requires OS X Mavericks 10.9.1 and Server 3.0.1.
Note: the update will only install if you have already launched Server app and used it to set up services.
Pre-orders and previews begin April 10, launches April 24.