This is a huge equalizer in the "apps vs. web" race. The beauty of this small change is that it will now make it as easy to use an app as it is to use the accompanying website. Currently, I'll choose to use, say, the Fandango mobile site over the Fandango app, even if the app is smoother, cooler, etc., because the mobile site autofills my login and credit card info, making a movie ticket purchase substantially faster. By adding the convenience of iCloud Keychain, apps once again have a leg up on websites.
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Sure, I'd love to see some cool "Samsung" features like water resistance, and changing photo focus in post when the new iPhone comes out, but CONTINUITY, that's the *****! That's why I love Apple. A powerful, building block improvement that will have a massive impact on the whole ecosystem. Apple doesn't chase trends and run around trying to find the next gimmick that will catch consumers fancy. They LEAD. It's not always obvious to everyone, and it sometimes requires a degree of faith, but not total faith. Because what little faith is required is always rewarded in smart, elegant, surprising and often game changing ways. They are methodical, which is what allows them to create an ecosystem that truly is more than the sum of it's parts. But too often, we all are guilty of getting distracted by the next shiny thing, and wish "our guys" would do something fun & cool like the "other guys" are doing. Well I, for one, am feeling pretty damn great right now about the horse I put my money on back in 1984. WWDC has restored the "faith" that has sustained me the past year or so (not the hugest fan of the aesthetic introduced in iOS7), and I can't wait the see how all these huge "little" changes play out in the next year.