A new setting added to OS X in Mountain Lion restores a classic Expose behavior for viewing all windows on the screen.
Apple's Missions Control feature in OS X is an evolution (or perhaps devolution, depending on your perspective) of the Expose and Spaces feature that was introduced in OS X 10.5 Leopard. While options for multiple desktops are great for organizing work, in some of Apple's decisions with the move to Mission Control it left out some of the simple approaches to Spaces that users became accustomed to.
One of these features is the ability to view every open window in one Expose view, regardless of what application it uses. This option was available in OS X 10.5 and 10.6, but with Lion and the introduction of Mission Control, Apple only allowed the windows of a single program to be presented in Expose view. Instead, the Mission Control interface showed windows for separate applications in groups, which was intended to offer a seemingly more intuitive approach for viewing all windows, but ultimately was not the same thing.
In Mountain Lion, Apple has changed this by adding an option that restores the ability to view all windows in Expose view. The default Expose behavior is still as it was in OS X 10.7, but if you go to the Mission Control system preferences you will see a new setting for grouping windows by application. If you uncheck this option, then invoking Expose will show all of the open windows when you enter Mission Control instead of clustering them in stacks.
Unfortunately there is no option to dynamically switch between this and the standard grouped Mission Control view, as both might be beneficial to have without requiring a settings change; however, this is not yet an option. If you prefer to have the Mission Control view be more application-centric then leave this setting enabled, but if you prefer to have it be window-centric then uncheck this option.