Yesterday I read through the 10.1 AppKit (Cocoa ApplicationKit Framework) release notes.

Here are the highlights, from my POV:

• NSStatusBar is the thing in the system menu that holds the custom menu items. NSStatusBarItem (I assume) is the class that represents A custom menu.
• Cocoa apps can have custom and dynamically generated custom menus from the dock!
• Keyboard navigation (see Keyboard Control panel)
• Ctrl-D for Don't Save in Save Panels
• Support for discovering when screens are added and removed when waking up from sleep
• Credits.html is looked for before Credits.rtf(d), so you can put links in your credits list in the About panel and they're clickable to the web browser.
• First Responder Action methods for speaking user-visible text
• Many NSDrawer bugs fixed.
• NSMovie now supports URLs for movie references (http:, rtsp:, etc.)