The difference between great vs. poor user interface design can be hard to quantify. Similar to the supposed difference between art and pornography -- you know it when you see it.

You certainly can't tell which is a better designed product simply by looking at a list of their features. Two products may have similar options, one may even have more options than the other, yet the lesser-featured product can be far superior -- because of its superior user interface design.

Well designed products work they way you expect. As you use such products, it becomes clear that the designers thought about the concerns you might have and designed the product to address those concerns. This may seem like something that happens in the design process of almost every product -- and thus not worthy of praise. In an ideal world, such would be the case. We do not live in an ideal world. Too often, I start using a product and almost immediately begin to grumble: "Did anyone even turn this thing on and try to use it before they started shipping it?"

Apple, of course, has built its success on a foundation of superb interface design. More than anything else, it is what has distinguished the Mac OS from Windows. Unfortunately, you can't always see the value of a well constructed interface from a brief look. This is one reason that Macs have sometimes had a tough time. "Why should I pay more for a Mac? A PC with Windows 95 has all the same features," a prospective buyer might have asked more than a decade ago. Sometimes I think there are two types of technology users: those who need to ask that sort of question but never "get" the answer and those who get the answer without ever having to ask the question. Fortunately for Apple (as well as for those of us who root for the types of products Apple makes), the latter group appears to be growing.

It is this recognition of superior user interface design that, more than anything else, accounts for the stunning success of Apple's iPod and iPhone. The arrival of these products sent a clear message: There is a better way! To give you an example, consider visual voicemail on the iPhone. Sure, every mobile phone supports voicemail. But after you've used an iPhone, and seen how easy it is to listen to any stored message, in any order you want, without having to first dial a number and work your way through a phone tree -- you begin to see how saying "the iPhone and Phone X both support voicemail" doesn't begin to tell the story.

Not all bathroom scales are equal. If you pay attention to user interface design, it will drive your purchase decisions. It made a decisive difference, for example, in my recent purchase of a bathroom scale. Based on online recommendations, I narrowed my choice to two scales. Unable to easily pick a winner in the store, I bought them both home to test them out.

Both scales needed to be turned on before you could get a weight. With Scale A, however, a tap of your foot anywhere on the scale's surface turned it on. With Scale B, you had to press a small button to turn it on. If you worked at it, you could press the button with your toe; but it could take several seconds to pull off this minor feat. The other alternative was to bend over and use your hand. It may sound trivial, but this is not want you want to have to do every time you weigh yourself. Unfortunately (for Scale B), this was only the start of the button pressing differential. Both scales offered the option to provide various metabolic data (such as your Body Mass Index, BMI). Scale A cycled through all the data after just one button press. With Scale B, you had to press a separate button to get each of four data points (making this worse, the labels on the buttons were hard to read from a standing position). Without hesitation, I returned Scale B, grumbling to myself: "Did anyone actually stand on this scale and try to use it before they let it leave the factory?"

The TiVo remote surprise. Sometimes, a well-designed feature is so subtle that it is barely considered worth mentioning -- even by the vendor. Yet, it can be critical. When you find such a feature, it's like discovering buried treasure. Such was the case with setting up the remote control of my new TiVo.

I was already familiar with TiVo's renowned user interface, so there were no initial surprises here. The surprise came when I began to configure the TiVo remote to work both with my TV and my audio receiver. In Step 1, you enter a 4 digit code so that the remote will turn on your TV and adjust the TV's volume. In Step 2, you enter a code for your receiver. After doing this, the remote's volume and mute buttons control your receiver instead of your TV. I did this and all worked as expected. Except for one problem: While the TiVo remote could turn my TV on and off, it had no such effect on the receiver. A primary goal for me was to eliminate the need for multiple remotes. If I still needed a second remote to turn the receiver on and off, TiVo had missed the mark.

Here, at last, was the surprise: TiVo had anticipated my concern and had an answer. As I recall, the TiVo onscreen prompts did not tell me about it (a minor flub on TiVo's part). Instead, I discovered it only after searching the TiVo Web site: After completing Step 2, return to Step 1 and enter the code for your receiver (it won't even be listed on the screen, you have to remember it). Now, the TiVo remote's TV Power button will turn on and off both the TV and the receiver simultaneously!

Surprisingly, most other remotes (even most universal remotes, aside from programmable ones such as Logitech's Harmony remotes) are not able to pull of this feat. At best, if you want a remote to switch from controlling your TV vs. your receiver vs. your cable box/DVR, you have to press a separate "device" button each time. If you forget to do this, it can take several more button presses to recover (such as when you accidentally change the channel on your TV instead of your cable box, and have to reselect the proper Input). In contrast, TiVo's remote sends signals to all three devices without any device switching required, each button on the remote always working the expected device(s). That's great user interface design. And it's a great example of why user interface design matters.