Gatekeeper is a new security technology Apple has released with Mountain Lion, which allows you to download and install apps from developers registered with Apple, regardless of whether those apps are available for sale on the Mac App Store or on the Web. If an app that has been signed by a registered developer misbehaves, Apple can disable that app and ban the developer from creating new software registered with Apple. Read more about Gatekeeper in our hands-on with the new feature.
With Mountain Lion, will Gatekeeper prevent my current software collection from running? Will I only be able to run apps I download from the Mac App Store?
No; you'll be able to open any software you choose to, though you can restrict this to Mac App Store-only purchases if you wish. In System Preferences -> Security & Privacy -> General, you can choose whether to run software exclusively from the Mac App Store; from the store and from non-Mac App Store developers who have registered with Apple; or from any developer, anywhere.
How will Mountain Lion's sharing features work?
Like iOS, Mountain Lion has a new Share button that allows you to send just about anything--website, picture, video, file, text excerpt--using a variety of services. Those services vary depending on the app: For instance, in Safari, you can share websites to your Reading List as well as via email, iMessage, and Twitter, while videos can be shared via YouTube, Vimeo, or AirDrop. If you right-click any text, you'll be able to share that via email, iMessage, or Twitter, too.
In addition, the Mail, Contacts, and Calendars pane of System Preferences has been updated so that you can add systemwide integration with Twitter, Flickr, and Vimeo, along with three other China-based services.
How does Mountain Lion integrate with iCloud? What's different from Lion's iCloud integration?
Building off Lion's basic iCloud integration, Mountain Lion will support Accounts sync, allowing you to take your passwords and preferences from one machine to another, along with Documents and Data. You'll also be able to sync your notes, and--while we haven't been able to personally confirm it--possibly sync your open Safari tabs, too.
Will integration between iCloud and Mountain Lion replace any soon-to-be-dead MobileMe features?
You are referring, of course, to Apple's plans to discontinue iWeb publishing, MobileMe Gallery, and iDisk in June. Mountain Lion's Accounts sync will transfer passwords, but otherwise, there are no major MobileMe feature resurrections.