Mission control is a great tool but with many programs and windows open, it can be difficult to identify what is in specific windows. Just as you do in the Finder to activate Quick Look, press the space bar when hovering your cursor over a window to zoom the window without making it active and quitting Mission Control. This is very useful if you're constantly working with a lot of small windows, or a lot of windows with text that all look the same in Mission Control.
[kirkmc adds: We ran this as part of a Mission Control tips hint last year. But that hint is actually incorrect; this does not "Quick Look [a window's] content," but rather zooms it. Quick Look is a very specific display which allows you to scroll through multi-page documents, view videos and more. If you use this to zoom in Mission Control, you cannot scroll; for example, if I zoom the BBEdit window in which I'm editing this hint, I can't then scroll up or down in the window, as I could if I Quick Looked the text file in the Finder.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20120329084425988