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Notifications are not new to iOS 5, but how it stores and displays them is. You can now see all of your notifications in one place by swiping down from the top of the screen, and display notifications in a different, more discreet way. If you are playing a game, it bubbles up from the top without interrupting your game play.
iMessage is Apple's equivalent to the BlackBerry Messenger service. So many people flock to the BlackBerry to have encrypted instant messaging conversations with their friends; well, Apple now has its own encrypted, secure messaging solution. In a very similar fashion to "BBM", you can send messages, photos, and other content -- and see when messages have been delivered and read.
Though only a small thing, and hardly a 'major feature', Apple has now put a camera button on the lock screen. To access it, you can simply double-press the Home button to enable the button. You can then take a photo even faster without having to enter a password. Your photos are still safe though, as it locks out your other pictures until you enter in the Home screen password.