Apple's Quick Look feature in OS X is a convenient way to preview the contents of a document before opening it fully in an application. To invoke Quick Look, simply select a file in the Finder and press the spacebar, and you will see a window appear that shows a view of the contents of the file, or for unsupported file types will show the file's icon along with some information about it.
Quick Look uses small plug-ins that provide it with a lightweight interpretation of a specific file format, so it can display some detail about the file to you. While convenient, the initial implementations of Quick Look in past versions of OS X have had problems with either displaying large quantities of content that you had to scroll through, or getting stuck when resizing windows when you switch between items, sometimes resulting in a small window being used to display a large image and making it difficult to see.
While these issues may still happen from time to time in Mountain Lion and while you can resize the Quick Look window manually to accommodate its contents, Apple's use of full-screen modes for applications makes managing these occurrences a bit easier.
When you invoke Quick Look, you will now see a small zoom option in the top right of the window, and as with any application that supports a full-screen view, when clicked this will bring the current Quick Look view to full screen (Quick Look's full-screen option is a little unusual in that it does not occupy its own space, but rather is an overlay to the current space).
In addition to the full-screen option, Apple has included a small shortcut for invoking full screen that can make navigating and previewing files with Quick Look much easier. With the Quick Look window open simply pinch-zoom on a multitouch trackpad and the Quick Look window will expand to full screen. If you do not have a multitouch trackpad, then you can hold the Option key when pressing the spacebar to invoke Quick Look immediately in full-screen mode.
For image files, holding the Option key down will display the images at their native resolution in full-screen mode, but you can release the Option key in this mode to have Quick Look zoom the image to the entire screen.
These full-screen options makes previewing some files' contents far easier, especially if they contain formatted text, which is common in Office documents.