The last option only allows apps downloaded from the Mac App Store to install or run. That's as self-explanatory, and as secure, as you can get.
Despite concerns from some that Gatekeeper goes to far, or doesn't go far enough, I like the options. They're a useful compromise for IT departments already accustomed to dealing with malware on the Windows side; once apps necessary to business become "Gatekeeper aware," so to speak, concerns about Mac malware or untrusted apps will be one of the last things on the mind of IT staffers. Granted, it will take some time for app makers to climb on board, but Gatekeeper should start that movement.
Mountain Lion's Security & Privacy preference pane now includes Gatekeeper, which is designed to head off malware by setting limits on which apps can run.
Share Sheets arrive
Apple has made social sharing in Mountain Lion more integrated with Share Sheets, which are built in to home-grown applications like QuickTime, Safari and Notes and allow you to easily share what you're looking at with others. (Third-party developers will need to rewrite their apps to incorporate the feature, but its usefulness should be readily apparent, especially for Twitter users.)
Every app's Share Sheet has a different set of functions, depending on context. For instance, clicking on Safari's Share Sheet button allows you to add a Web page to the Reading List, to your Bookmarks, or to share it via email, message or tweet. Interestingly enough, Facebook isn't an option in most apps. You won't find Facebook under the System Preference for Mail, Contacts & Calendars (where you will find Twitter). But if you look under the Share menu in a QuickTime window, you can share your movie to FaceBook. The same is true in iPhoto's Share Sheet.
Given the explosion in social media, it's a smart move to incorporate easy ways to share digital content. I'd expect to see more of this as OS X develops.
Odds and Ends
Game Center: Apple has also added Game Center to Mountain Lion. Game Center is a centralized location that on iOS devices provides easy access to global game score and leader-board tracking; it also allows you to see what games friends are playing and how well they're doing, among other details. Game Center's support for turn-based and head-to-head games now comes to the Mac, allowing you to play supported games on your Mac against people playing on iOS devices.
Notes: In Lion and earlier versions of OS X, the Notes app was always bound to Mail; but in iOS, Notes has been a standalone app. When Mountain Lion is released, Notes will become a standalone application with an interface lifted from the landscape view on the iPad. (Similarly, to-do's and reminders get the standalone app treatment, too, with their appearance drawn from the iPad.)
Safari: Apple's Web browser gets a few tweaks, too, including a new address bar that handles searches as well as url addresses. Safari also gains a "Do Not Track" option, Apple's response to growing privacy concerns among online surfers.
Software Update: In Mountain Lion, software updates are now handled through the Mac App Store, and Notifications appear when updates are available for either the operating system or apps downloaded from the Mac App Store. This will surely raise awareness of the App Store and the apps it offers.
Like Lion before it, Mountain Lion continues the merger of iOS and OS X features in a way that helps provide a consistent experience across Apple devices. In concert with iCloud, Apple is moving to make things easy to use no matter which device you have in hand, an iPhone or a iMac, an iPad or a MacBook Air. The main difference revolves around each device's interaction method: mouse and trackpad for the Mac; touch screen gestures on the iPad and iPhone.