iOS 5 adds lots of big new features, but also many more small tweaks like the ability to mark email messages as flagged or unread.
When reading an email, the word Mark appears in small blue text to the right of the date and time. Tap on it and options come up to flag a message or mark it as unread. Flag literally puts a tiny red flag icon on the message to help you find it later. Mark as Unread puts the little blue dot back so you can easily find and re-read it again.