Things that behave differently should look different. (Alan Cooper)

There are two things going on here. First, there’s affordances. That’s just a fancy way of saying what a thing does should be obvious from the way it looks. An affordance is just the little thing that tells you what that thing can do.  

Think of a door. If it has a push plate, it tells you you have to push the door to open it. If, instead, it has something you can grab, that tells you you’re going to be pulling this one toward you. And if it has a fairly standard-looking handle, that means that you need to grab and turn it before you can do anything else.

In a more digital context, radio buttons say, “Click one of me.” Checkboxes say, “Click as many of me as you like.” Sliders say, “There’s more here.” Links say, “I’m going to take you to a new page.”

That last one is actually a good example of what Cooper is talking about here. A very traditional standard for links is blue and underlined. A lot of sites, however, get a little creative. They might, for example, ditch the underline, or use a different color, or both. In that situation, though, it’s much easier to be confused by the link’s affordance. A user might, for example, confuse a link with bolding, or a title, or nothing in particular, or what have you.

The other thing going on here is consistency. In fact, a typical corollary to this quote is, “Things that behave similarly should look similar.” 

Now, the whole point of consistency – and standards, which help deliver that consistency – is reducing cognitive load. In other words, don’t make me think! So, if users have already learned a particular affordance elsewhere – on the web, on your site, in life in general – they don’t need to learn something new. 

Just to make this concrete, I once tested a mobile app that had a lot of inconsistency. Interestingly, though, this was mainly an issue of location. For example, the Submit button was on the bottom right, the bottom left, the top right, and the top left. The app was also pretty inconsistent when it came to terminology. Submit, for example, might be “Submit,” or “Done,” or “Complete,” or just-see-your-thesaurus.

So, it’s really not just about affordances. There are actually all sorts of ways to be inconsistent.

And if you have a door like this,

you’re going to need instructions to tell people what to do

Alan also said:

• "Creating a good UX is not a design problem, it’s a power struggle."
• "The fanatic preparation for the possible has the inevitable consequence of obscuring the probable."