You may have noticed that certain parts of a message in Mail can be highlighted when you hover over them, such as dates, times and addresses. For a while now, users have been able to select this information and add it to either Address Book or iCal without leaving the Mail application.
In Lion, there is now added iCal functionality that allows the user to edit the event that has just been created—all within Mail. Along the same lines, one can now preview webpages without having to open a separate browser! In either case, there is an option to then open the application separately, but having this added integration makes workflow a lot more seamless.