Photographers know this, of course. But for the few non-photographers who read my blog (and possibly even the photographers!) this might be interesting.
I took these two photos exactly 30 seconds apart. For the first one, above, there was just the right amount of light filtering through the clouds to provide some shape and contrast to the scene. In the second shot, below, the cloud has moved over the sun and the light has gone flat. Both of these photos are pretty nice, but to me the one with the shaping light and subtle shadows is the superior one.