Earlier, I blogged about one of my flash diffusers, and about how most flash macro photography is improved by softening the flash's harsh artificial light. My observations were not novel, of course, and I love spying on the various contraptions macrophotographers invent as they aim for perfect diffusion. See, for example, recent posts by Seth Burgess and Ted MacRae.
I'm now going to play the contrarian. For many images, the quest for ever softer, ever more even light leads to images that are ever so slightly more... dull. While we want enough diffusion to avoid blown-out highlights and completely black shadows, flattening the light too much can have a similar flattening effect on the image's personality. Thus, even when striving for clean, even lighting I often prefer one side brighter than the other, or, as for the spider above, just enough glint on the eyes to suggest a little mischief.