Web development revolves around starting and latching onto trends, and the flat design dominance won't be going away anytime soon.
What is Flat Design?
Minimalist design with simple, clear interfaces and flat illustrations are the hallmarks of flat design. Menu navigation is often paired with an icon-like graphic for quick visual identification of the buttons' function. The buttons or navigation elements are often identified by bright colors which separate them from all content and backgrounds. Think the Windows 8 app approach with plenty of open space filled by a quick-loading background image.
The Advantages of Keeping It Simple
Most designers in web development have moved towards eliminating unnecessary decoration or unintuitive menus. This trend has the advantage of being simple to grasp by a wide variety of users. More than anything, a user's eye is guided towards the navigational elements regardless of their familiarity with computers or the web.
Perhaps the largest benefit for website design is the responsive web design synergy. The design choices inherently avoid any incompatibilities with mobile devices, the buttons are easily accessed on any size screen, and the minimal use of images makes for extremely fast loading on 3G or 4G networks.
In 2015, we're going to see more innovation in flat design with button-hovering, scrolling layouts, and more!