Mission Control is a great new feature in Lion, however as it scales down all open apps and windows on your screen, it’s not always easy to clearly see which document is which.
One hidden feature in Lion is that you can press the space bar when hovering over one of the small window previews in Mission Control to zoom in on that window optimising the Quick Look feature so you can take a closer look! (Wait until the desired window preview has the blue highlight around it before pressing the space bar).
Initial Mission Control Window:
Using QuickLook within the Mission Control Window (Notice the Preview document has zoomed in) :