In iOS 5, you see all your recent notifications by pulling down from the top of the screen to reveal Notification Center. In Mountain Lion, the Notification Center list is a narrow band that lives just to off the right side of your screen. You can reveal it either by clicking on the new Notification Center icon at the far right of the menu bar, or by swiping with two fingers starting at the far right edge of the trackpad. Either way, your Mac's entire screen will slide to the left, revealing a list of what's been trying to get your attention recently.
There's also a new Notifications pane in the System Preferences app, analogous to the Notifications submenu in iOS's Settings app. From here you can choose which apps appear within Notification Center and how their alert bubbles behave.
Gatekeeper blocks apps... the first time
When Apple introduced the Mac App Store, the rumblings started: A lot of people wondered if the Mac was headed for an iOS-like future, one in which only Apple-approved apps could run on the Mac.
But with Lion and now Mountain Lion, those fears haven't become reality. You can still run third-party apps to your heart's content. However, with Mountain Lion, Apple is introducing a new feature called Gatekeeper that allows users to choose for themselves what kinds of apps can be installed on their Macs.
Right now, OS X checks an app the first time it launches, and displays a warning. It's an attempt to prevent malware apps from launching when you never intended them to. In Mountain Lion, that feature has been extended and tied into a new setting in the Security & Privacy pane of System Preferences.
By default, Mountain Lion will only let Mac App Store apps and Apps from "identified developers" launch for the first time. To become an "identified developer," Mac developers have to register with Apple and get a personalized certificate, which they then use to cryptographically sign their apps. Apple doesn't do any sort of background check on the developer, and it doesn't see any of the software.
Apple says that although these apps aren't as safe as Mac App Store apps, they're safer for a couple of reasons. First, a signed app can't be modified (to add in some spyware, for example) without breaking the signature. By default, Mountain Lion will refuse to launch an app modified in that way. Second, if it turns out that an app from a particular developer is actually malware, Apple has the ability to revoke that developer's license—at which point no future Mac users will be able to install software from that developer.