A few of my favorites in iOS 5 are...
The new Notification Center may be favorite of all the iOS 5 additions. With the new Notification Center you can keep track of them all in one place...like calendar appointments, new email, friend requests, tweets, and more. Just swipe your finger from the top of the screen to the bottom to access it.
Camera. Capture your moments in a moment's notice.
The changes to the camera are phenomenal. The best part is that you can open your camera from the lock screen. Other features are grid lines, pinch to zoom in, single-tap focus, and the best one...press the volume up bottom to snap the picture.
Safari. Better sight-seeing.
Tab-browsing...need I say more. Tabs have been a long time coming for Apple devices. Now with the click of the + you can have multiple web pages open at one time and switch between them with ease.
iMessage. iPad and iPod touch join the conversation.
You can now send unlimited text messages via Wi-Fi or 3G from your iPad, iPod touch or iPhone to anyone with those devices. Delivery and read receipts are part of the new message feature.