I think that common sense goes a long ways in UI design… But not all the way. It’s a learned skill like any other. I was reading up on calendar controls today (which I’m obsessed with, by the way) and caught a great post by the curmudgeonly Jakob Nielsen. Here’s the snippet I like best (he’s describing the top application design mistakes):

“Affordance” means what you can do to an object. For example, a checkbox affords turning on and off, and a slider affords moving up or down. “Perceived affordances” are actions you understand just by looking at the object, before you start using it (or feeling it, if it’s a physical device rather than an on-screen UI element). All of this is discussed in Don Norman’s book The Design of Everyday Things.

Perceived affordances are especially important in UI design, because all screen pixels afford clicking — even though nothing usually happens if you click. There are so many visible things on a computer screen that users don’t have time for a mine sweeping game, clicking around hoping to find something actionable. (Exception: small children sometimes like to explore screens by clicking around.)

Drag-and-drop designs are often the worst offenders when it’s not apparent that something can be dragged or where something can be dropped. (Or what will happen if you do drag or drop.) In contrast, simple checkboxes and command buttons usually make it painfully obvious what you can click.

Common symptoms of the lack of perceived affordances are:

• * Users say, “What do I do here?”
• * Users don’t go near a feature that would help them.
• * A profusion of screen text tries to overcome these two problems. (Even worse are verbose, multi-stage instructions that disappear after you perform the first of several actions.)

When I tested some of the first Macintosh applications in the mid-1980s, users were often stumped by the empty screen that appeared when they launched, say, MacWrite. What do I do here, indeed. The first step was supposed to be to create a new document, but that command was not shown anywhere in the otherwise highly visible Macintosh UI unless you happened to pull down the File menu. Later application releases opened up with a blank document on the screen, complete with an inviting, blinking insertion point that provided the perceived affordance for “start typing.”