iOS 7 is codenamed "Innsbruck," and will use a "very, very flat" design, sources for 9to5Mac claim. The OS is reportedly jettisoning any signs of gloss or skeuomorphism, making it flat on a level nearing the look of the Metro interface used in Windows Phone and Windows 8. Apple has sometimes been criticized for using unnecessary real-world analogies in the look of iOS and OS X, for instance by making its Calendar apps resemble leather daybooks.
The person most responsible for Apple's skeuomorphism after Steve Jobs, ex-iOS head Scott Forstall, was fired last year however, and now software design is being overseen by Jonathan Ive, who also controls the look of Apple's hardware. Ive is believed to be staunchly opposed to skeuomorphism, which would explain the radical shift in iOS 7's aesthetics. These are said to include new icons, toolbars, tab bars, and other overhauled interface elements.
Despite this, the learning curve for the OS is allegedly small, since core functions work about the same as they do in iOS 6. Apple has been testing ways to add more "glance-able" information to iOS, in the vein of Notification Center, but it's unclear if any that testing will see fruit in the new OS, which is scheduled to be shown off at WWDC this June.
One source claims that iOS' shift away from skeuomorphism will be reflected in the updated Game Center, which is even said to resemble the WWDC 2013 logo in some respects. If so, the logo may hint at the overall direction of Innsbruck.