The user can set Mountain Lion to be broader or narrower with the list of apps it's willing to launch. There's an option to allow only Mac App Store software to run, and an option to allow any app to run. The latter option is the equivalent of what's been the case in all previous versions of OS X.
For a more in-depth look at Gatekeeper, read our Hands on with Gatekeeper story.
Sharing and Twitter
Mountain Lion introduces an interface element inspired by iOS—Share Sheets. They're a pop-up menu that appears when you click on the Share icon in an app. Apple has implemented Share Sheets in several Mountain Lion apps, including Safari, Preview, and Notes, and developers can add them to their apps as well.
A Share Sheet provides a quick way to share whatever you're working on—a photo in iPhoto, a webpage in Safari, a document in Notes—with other services. If you share a webpage from Safari, you can choose to insert it (or just its URL) in a new Mail message, or insert a link in a new message in Messages, or even compose a tweet containing the URL. From Preview, you can choose to email the document you're viewing, send it via Messages, tweet it via Twitter, upload it to Flickr, or transfer it locally via AirDrop.
Most of these aren't really new functions. What's different is that Apple has centralized them and given developers access to this element, which presumably will lead to a more consistent sharing interface in future Mac apps. If that sounds familiar, it is: This is once again an example of the Mac taking a page from iOS, in this case from the Share button that's found commonly throughout iOS.
In most contexts, Share Sheets will include a Twitter option. That's because Mountain Lion is joining iOS 5 in adding system-level support for the popular communication service. You can add your Twitter account information in the Mail, Contacts & Calendars system preference (which is just dying to be renamed to Accounts). Once that's done, it becomes easy to quickly share items from just about anywhere via a Share Sheet. Select Twitter and a small floating composition window appears, allowing you to write and send a tweet quickly, without leaving the app you're working in.
Twitter integration doesn't stop there. You can also use Twitter to populate the avatars of friends in your Contacts list with their Twitter profile pictures. (Yes, Address Book has been re-named Contacts in Mountain Lion to match its counterpart app in iOS.) Tweet notifications can also optionally appear automatically in Notification Center.