"Flat" has become very popular.
This particular design trend found its footing in Microsoft's Windows Phone 7 design. It was all big, buttons and bright colors with no extraneous texture or ornamentation—ideas that later spread to Windows 8, iOS 7, OS X Yosemite, the Android L release, and many apps running on those platforms. Software is no longer focused on making onscreen content look like real-world objects. Our screens aren't made of felt or paper, so why should they look that way?
That said, there are plenty of things about older designs that new designs haven't figured out. We put our collective heads together and came up with the following list of modern UI design trends that get our blood boiling. We're sure there are more things that could make this list, but we're trying to keep our blood pressure down. If you've got some hate to spread around, kindly deposit it in the comments section.