File renaming in the title bar
It's a small thing, but a welcome one. Apps that implement OS X Mountain Lion's revised document APIs can have their documents be renamed in situ. No longer must you close the document to rename it in the Finder. Just click the document name in the title bar and enter the new name. That's it!
There are other controls available as well from the adjacent pop-up menu, but those were introduced in OS X Lion.