In my post Deconstructing the iPod Shuffle UI, I talked a bit about the notion of a limited UI where you really only do one thing -- in that case, click the button on the headphones.
Every now and then, I rediscover the infrared remote that goes with my iMac, and realize that for many mundane tasks, the remote does everything I need. (Mira lets me assign different actions for the remote button for specific applications. Candelair works around a Snow Leopard issue with Mira.)
For example, with a new album in a photo library, you might want to Next-Next-Next through each photo, and if one is particularly bad, trash it; if one is particularly good, mark it with a high rank or a coloured label; otherwise, leave it as-is and proceed to the next one. The up/down/left/right/play buttons on the remote can accomodate those actions comfortably. All the photo software offers keyboard shortcuts for these actions, that can be assigned to the remote buttons. So this brain-intensive task can be accomplished while sitting back and taking in the view on a large screen.
One thing had been bothering me about the UI for the iPod Touch. If I'm listening to music with the screen turned off, say in the car or the office, and I'm interrupted (want to stop the music suddenly) or annoyed (want to immediately skip to the next song), normally this requires turning on the screen (Home button or power button), and unlocking the iPod by swiping before the music controls are available. I should have realized Apple would bring the "single-action UI" to the rescue. Instead of unlocking the whole unit, once it is powered on in a locked state, 2 more presses of the Home button bring up music controls that let you pause or skip without unlocking. It's almost iPod Shuffle-like. Now if only there were a way to assign star ratings without unlocking.
Many other day-to-day activities could benefit from this same kind of UI, in particular with the remote. You've got a lot of objects to consider or evaluate, and a limited number of actions for each one, including a default of "do nothing at all". Someone could play through a set of songs and give high / medium / low rating to each one, or trash it. A programmer could scroll through a list of bugs or functional specs, and for each one indicate "closed", "waiting for more information", "this does not apply to me", etc. That may be the way of the future, where carpal tunnel and hunched shoulders are just a distant memory.