The best way to get a handle on this question is to analyze a common interface, but instead of exploring the graphical interface of some app or web site, let's compare the keyless entry systems for the Nissan Leaf and the Toyota Prius. The basic idea of keyless entry is that you can lock and unlock your car without getting out your key or fob. It sounds like a simple task, but simplicity can be much harder than it appears. Even seemingly obvious designs are very easy to get wrong.
A Tale Of Two Keyless Entry Systems
Both the Leaf and the Prius have a keyless entry feature that purports to allow the driver to get into the car without rummaging around for a key to unlock it, but they accomplish this feat in slightly different ways. The Leaf has a physical button on the driver and front passenger door handles right where your thumb rests when you reach for the handle. If you have the key fob near the door, you can push the button once to unlock that door. Push it again to unlock all of the doors. Push it a third time to lock all of the doors again. Pushing the button after opening any door will also lock all of the doors.
In comparison, the Prius has a sensor on the inside of the driver door handle that can tell when a hand crosses it. If the key is near the door when this happens, it will quickly unlock before the driver can pull the handle to open the door. To lock all of the doors, there is a pressure-sensitive area on the top front part of the door handle that acts like a button.
The first concern for any design is to get it working. If it doesn't function at a basic level, then there's no point in looking any further. The feature is broken and no amount of beautiful design will save it. Both of these keyless entry systems work flawlessly in the common case, so there's no issue with basic functionality. You can walk up to the driver side door with the key in your pocket and unlock the door with the door handle. It just works.
If you start expanding the situation a bit, though, the Prius design starts revealing problems. Say the temperature outside drops below freezing - not an uncommon occurrence here in Wisconsin for four months out of the year. If you're wearing gloves any thicker than normal driving gloves, the sensor won't recognize your hand and the door won't unlock. You have to pull off a glove and either grab the freezing cold door handle or hunt for your key fob to unlock it, defeating the purpose of keyless entry.
The Leaf's button is nicely rounded and set out far enough from the rest of the handle that you can easily mash it with even the thickest winter gloves on. It's a nice advantage to be able to keep those gloves on while getting into the car. Once inside, the Leaf's essential buttons on the console are large enough to manipulate with thick gloves on so you can leave them on and drive, but I digress.
Things Diverge From Here
Expanding our situation even further, let's say you have a kid. Many people have these, and for quite a few years they need to or like to be carried. The contortions involved in carrying a kid and getting at a key in the wrong pocket (it always seems to be in the wrong pocket) could inspire professional gymnasts. To avoid struggling for your keys with the Prius, you have to open the driver door, twist and bend to hit the inside door unlock button, and then go open the door where the child's car seat is - probably on the other side of the car. You can't double-trigger the door handle sensor to unlock all of the car doors, and the sensor only exists on the driver door.
With the Leaf, you can slide to the left, hit the door unlock button twice with a free knuckle, slide to the right (reminds you of being at a wedding reception, doesn't it?), and open the child's door. If you're on the passenger side, reverse left and right and it works just as well because there's a button there, too.
If you're not carrying a kid, this situation still applies if you're carrying anything heavy that you want to drop in the back seat, but what if you want to put that heavy object in the trunk? If you have a Leaf, no problem. Go right up to the hatch, maneuver one of your hands so you can pull up on the hatch release, and the car will unlock the hatch and pop it open for you. Drop the heavy object in and close the hatch. You could walk off without worry at this point to get more heavy stuff because the car is still locked!
Now try the same thing with a Prius. Sorry, you have to go over to the driver door, get your hand around the handle while carrying that heavy object, open the door, find a way to contort yourself to hit the door unlock button on the inside, and go back to the hatch to open it. That's right, the hatch does not have the keyless entry feature. If you don't want to look like an idiot pacing around your car with a heavy package, you'll have to find a way to get at that key in your pocket. Don't forget to lock up when you're done because the whole car is unlocked.
How Did Nissan Get It So Right While Toyota Got It So Wrong?
There are many more situations that routinely come up where the Leaf's keyless entry proves to be so much more convenient than the Prius. Why is that? What made key fobs so nice was that you could unlock all of the car doors from anywhere near the car. The person unlocking the car wasn't necessarily going for the driver door. Nissan must have realized this and designed the keyless entry system so that the ability to unlock any door was within easy reach. That makes keyless entry work in almost every situation.
Toyota seems to have gotten distracted by new technology that involves extra sensors that probably play nicely in a dealership showroom with potential customers. And while having a door handle that senses your hand is cool at first, the novelty quickly wears off when you start running into all kinds of situations where the feature is essentially useless. Every time that happens, I get frustrated at the system's inability to meet my needs, and that adds up over time.
When designing an interface, first and foremost it has to work. Both keyless entry systems achieve that. Then the interface should be designed for as many use cases as possible. Nissan nailed this goal, and Toyota came up short. Once those requirements are met, you can design for beauty, aesthetics, and high-tech coolness. Nissan settled for a simple, functional design with utilitarian buttons, and they could improve on this design in the future if they figure out how to solve the not-so-simple sensor issues.
Toyota succeeded in making a sleeker door handle, but you'll end up pulling out your key fob a lot more with a Prius. They misunderstood what makes a good keyless entry system, so almost every case where you really need the system to work, it falls flat. If Toyota would have used their keyless entry design in a wider variety of situations, they likely would have noticed its deficiencies and come up with a better design that would be useful when it counts. Don't make the same mistake with your own designs. Use them in as many real-world situations as you can think of, and make sure they work as simply and conveniently as possible.