With OS X Lion, Exposé merged with Spaces and became Mission Control, as a result the behavior of one of many peoples favorite window management features changed; you could no longer see all windows from all apps in a single screen. Instead, app windows were grouped by application and stacked atop one another. That incredibly useful old Expose feature of seeing all windows is back in OS X Mountain Lion though, and you just have to enable it to see everything from a single screen again:

Open System Preferences from the  Apple menu Click on “Mission Control” Uncheck the box next to “Group windows by application” Do a three-fingered swipe up (or hit F9) to see the traditional Expose-style window manager

With Mountain Lion, you can also continue to perform the app-specific Expose style window view by hovering over the apps Dock icon and performing a three-fingered downward swipe.

Mission Control is very powerful, check out 9 great tips for Mission Control or view our archives on the topic for more.