Applescript and GUI in Mavericks
One of the big changes with Mavericks is the enhanced security and this has an impact on all developers even those who write a “simple” applescript to avoid a tedious or repetitive series of actions.
Developers have often relied upon AppleScript’s ability to control the user-interface, to provide an automation solution when no direct scripting support of an application or process was available. While this valued ability continues to be fully supported in Mavericks, the enhanced security focus of the new OS requires a few changes in how scripters access and apply the Accessibility frameworks.
There is a really comprehensive discussion of Accessibility Preferences and GUI Scripting over on Mac OS X Automation. Well worth a read.