In the new Reminders app, you can select several reminder lists by clicking on one, then Command-clicking on others. When you do this, the reminders will all display in one window, with a header saying, for example, 3 Lists. A smaller header will show the name of each list, with each reminder under the header for its list. If you then click the Hide Reminders button at the bottom-left of the app, you get a very neat reminders app that shows all non-completed tasks in a single list.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20120728043121380