One of iOS' biggest drawbacks to date is its horrible notification system. You know what I'm talking about, you're busy tapping away an email (or working on that impossible Angry Birds level), and BLING!, up pops a modal dialog with a text message, tweet, whatever, smack in the middle of your screen.
Your options at this point are to either a) Close the window and hope that you remember to check it again later, or b) View the notification and interrupt what you're doing. While there are worse things, it's annoying, inconvenient and promotes closing (translation: forgetting) notifications.
It gets problematic when you get multiple notifications. Since they're confined to the same bulky notification window -- which obscures your screen -- they get truncated and only display the sender and the number of pending messages. Completely useless and borderline offensive.
iOS 5 does away with the insanity that is iOS notifications as we know them now. Apple's replaced the jenky implementation above with a new Notification Center below -- and it's a welcome breath of fresh air.
The interface above is available by swiping down from the top of any home screen and similarly put away by swiping upward via the little handle at the bottom center. It even includes a stock ticker and the current weather forecast.
When you receive a notification in iOS5 it now appears subtly at the top of the screen -- instead of blocking the middle of the screen like the current notifications do.
Touching it whisks you to the appropriate app (Mail, Messages, Twitter, etc.) and after a period of time it goes away. If the new Notification Center UI looks familiar to the jailbreak app MobileNotifier (below), that's because Apple hired Peter Hajas (its developer) in May to help address the problem.
The icing on the cake is that iOS 5 notifications extend to the lock screen too, so you can see them immediately without unlocking your phone. They even wake the screen briefly to show that a new notification has arrived. Swiping "slide to unlock" takes you to the app that generated your most recent notification.