Interaction design of the century you ask? The iPhone? Nope. Square payment thingamajob? Not even close. Gmail? Closer. The Undo Send feature in Gmail? Spot on. In a carpet bombing of features and half hearted engineering experiments, Gmail, completely accidentally if you ask me, stumbled upon one of the most niche and most realistic human attributes to address in a bit of machine. Regret.
I saw a talk by some PhD guy once who was studying how people use two monitors with a mouse and did loads of measurement on it. Adjust it like so, you save a couple milliseconds. Imagine, five years of your life spent studying how long it takes someone to adjust to using two screens side by side. Yet, this is something that doesn't matter half as much in anyone's life as Undo Send. Not half as much as despair or exhaustion. Where's the Undo Condescension button? Or maybe the I Know We Work Together But I Don't Really Have Time for Your Bullshit button?
Well this I suppose is the problem really, concentrating on the technical forest and not the realistic trees. Thinking too much about the niggly, little, practical things that slightly suck that humans invariably get used to instead of the elephant sitting on your lap between you and the screen that really affects daily life. Ultimately though, its about forgetting that we're not machines, we're human dammit, and we have feelings, and a lot of times these feelings trump all the milliseconds in the world saved.