iOS 7 is the biggest change to iOS since it's release. The mobile OS has been completely redesigned from the ground up by Johnny Ive himself, introducing a very flat and colourful design.
It did however concern many designers because the design seemed to very child like, inconsistent and sometimes very poor when it comes to the UX side of things. But then a lot of people love the new design - it's all down to preference.
There have been a number of iOS 7 redesigns since it's announcement, but this is easily my favourite. This concept made by Alexey Masalov is very radical when it comes to aesthetics. He has changed the style of the iOS as a whole, introducing a consistent base colour with the icons and wallpapers adding colour.
Apps have been completely redesigned too, they're much more minimal and really do put the content first. It'd be impossible to tell what it'd be like from a UX perspective because you'd have to use it, but the design is downright awesome.