User Experience is really all about delighting your users. You want them to accomplish tasks with ease and not encounter any roadblocks that are a direct result of your design. Many of the delightful things about an app or interface go unnoticed because they are the tiniest of features. These microinteractions can set the tone for your users and dictate the feel and performance of your design.
Dan Saffer is an expert on microinteractions. In fact, he wrote the book on it. He says that microinteractions essentially operate based on triggers, rules, feedback, loops, and modes. For example, when you engage a scrollbar, how fast does it scroll? Or when you click a volume up button, what percent increase is each click?
Just think of a car. In the broadest terms, a car is a car. But the styling of the interior, leather seats, placement of cupholders, and how the in car stereo system works all help differentiate one car from another. These are often subtle differences, but as with microinteractions, these small differences are crucial to the overall feel and experience.