If you're using Quick Look on an image, you'll appreciate the following trick for zooming.
Once you've opened the QL window on a JPEG or other image file, simply press Option and the pointer will change to a magnifying glass with a plus sign in the middle. Click somewhere on the image to zoom in.
Drag the cursor around to move around, or use the scroll wheel (2 fingers on trackpad) to move around. Option-Shift-click to zoom back out.
[crarko adds: I tested this, and it works as described. It doesn't work on PDF's, which is a bummer.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20100717105329814