Never use a single element or style as an indicator. It’s a very basic principle, but one that’s often forgotten (yeah, I do it too). To take a small example, say you’ve decided to turn off underlining on active hyperlinks, and just use color to indicate the links. Now you’ve possibly screwed color-impared users, or users with a high-contrast theme.
Good user experiences, however, don’t rely on single clues to differentiate content, indicate actions, etc. Here’s a great example. On iPhone OS 3.0, OmniFocus gets horked, dropping the text labels on three of the six categories:
Had they only used text, I’d have to memorize the list and think about the ordering every time*. But, since each group also has a unique color, I lose no productivity, even though the UI is broken. This is the sort of good redundancy I love to see. Even if was an accident, it’s a definite WIN.
*=Yeah, yeah, there are only three things. But what if there were 30, or the ordering changed dynamically?