Agile practices have proven themselves time and again for development and evolution of software systems. But, it's not clear if the same agile approach can benefit user-facing aspects such as public APIs, user interface design and user experience. If you change your API constantly, no sane developer will use it. If your user interface design or experience keeps shifting users will get confused and angry that they have to face a new learning curve whenever you decide to make a change. Sometimes, users will be upset even if the changes are demonstrably beneficial, just because of switching costs. Remember users didn't subscribe to your agile thinking and are just interested in using your API/product.
What's the answer then? Do you have to be prescient and come up with the ultimate API and user interface right at the beginning? Not at all. There are several solutions that will allow you to iterate here as well. But, you have to realize that iteration on these aspects should and will be slower and more disciplined.
Possible approaches include A/B testing, keeping the old API/interface available, deprecating previous APIs, backward compatibility, testing rapid changes on groups of users that sign up for beta. In general, the more successful you are, the less free you are to get rid of legacy. Probably the best example is Microsoft which still allows you to run DOS programs on the latest Windows versions and used a variety of approaches to iterate on the Windows desktop experience, including handling the frustration from users whenever a new version of Windows comes out. Windows 10 is a fine response to the harsh criticism Windows 8 endured.
Agile programming, user interface design, Agile Software Development, UI/UX designers