Separation: Light subject vs. darker background
In this image, the lady walking is clearly distinguishable against the background. In fact, I waited for that very moment to take the shot, as I wanted her head and face to contrast against the background. Since I was range-focusing (i.e., using a small fstop to ensure that a maximum number of possible subjects would be in focus, regardless of their distance from me). Although useful, this technique also brings far too many distractions: foreground and background tend to be a long continuation from another, making it hard to separate the subject from its environment.