Lifehacker has put together a list of what they call the "top 10 secret features of Mountain Lion." That's not quite right -- these features aren't secret, because it would be pretty silly to release an OS update and not tell anyone about it. But they are subtle, and this is a nice list of features you may not have noticed yet.
For example, the fact that you can tweet right from the top of Notification Center, or that you can share photos straight from QuickLook using the Share button. You can also rename files in TextEdit (or share them to iCloud) just by clicking on the document's name at the top of the window, and you can even insert pages into PDF files in Preview through the Edit menu. Helpful tips, for sure.
We'll even add a few more features in that you may not have noticed yet: Over in Launchpad (which you can access from your dock or pressing F4 on a new Mountain Lion install, or set up a hot corner for), just typing will start you on a search for any app you've installed through the Mac App Store. And you may not have noticed, but Apple's added some brand new Screen Saver and slideshow options for when your Mac goes to sleep. Just like Apple's other OS releases lately, there's a lot of new stuff to go find and play with in Mountain Lion.