Embrace flat design
In Bootstrap 3, it begins to embrace flat design. First from the official webpage of Bootstrap 3, we can find elements are flat designed. Also, all buttons have no box shadows now, they are flat and simple. But the flat design is not complete, in some components, we still can see box shadows such as in drop down menus. Not sure whether they will adopt complete flat design in the final release of Bootstrap 3, but it should be the trend.
In Bootstrap 3, mobile device support is built into the library:
With Bootstrap 2, we added optional mobile friendly styles for key aspects of the framework. With Bootstrap 3, we've rewritten the project to be mobile friendly from the start. Instead of adding on optional mobile styles, they're baked right into the core. In fact, Bootstrap is mobile first. Mobile first styles can be found throughout the entire library instead of in separate files.
With the responsive design, your pages can be displayed on different devices properly...