A new book about embattled Yahoo CEO Marissa Mayer reveals her three rules of great app design. Written by Nicholas Carlson, Marissa Mayer and the Fight to Save Yahoo! details everything from Mayer's early (and misguided) attempts to play Steve Jobs at Yahoo to her contentious decision to change the Yahoo logo. Though she's made some missteps, Mayer has also seen triumph, especially in the design of new apps like Yahoo Weather and Flickr. Here are three rules that informs all of Mayer's decisions when it comes to app design.
• The Two Tap Rule — Mayer has something called the two-tap rule of app design. The test for the rule, Mayer says, is simple: "Once you're in the app, is it two taps to do anything you want to do?" If no, time to redesign the app." The Yahoo Flickr app uses this to great effect: from the opening screen, you can take a picture, surf your screen, navigate albums, check out groups, set alerts, and more, all with just a couple taps.
• The 5-Point Rule — Back in her days at Google, Mayer would tell designers to count a point for every different font, font size, and color on a page. If a page goes above five points, it's time to redesign. (I assume this doesn't include Google's logo, which by this logic counts as 5 points all by itself.) And as my editor points out, it's too bad Mayer hasn't brought the 5-point rule to Yahoo's homepage.
• The 98% Rule — Mayer believes that every product should be designed for the way it will be used 98% of the time. As an example, Carlson says, Mayer will often point to a Xerox copy machine. You can do all sorts of things on a copier, but all 98% of people really want to do is make a single photocopy, which is why using a Xerox is as easy as slapping your paper on the glass and hitting a big green button. According to Mayer, every app should have a button like that. Flickr is a good example here: although Flickr is a huge social network based around photography with a million different features, there's a big, bold camera button right at the bottom of every screen.
What's salient about all these rules is how they boil down relatively complex UI/UX principles into terms that anyone can understand and even act upon.
You can read more about Mayer's design principles over at Business Insider.