I think a lot of people don't appreciate the importance of icons enough. Icons are the apps' faces, their metaphor. We identify apps with their icons and we are drawn to them when they are good. It goes beyond the home screen, they are everywhere on iOS: notifications, settings, the App Store and the app switcher. For that reason, icons should ideally meet two criteria: distinctiveness and descriptiveness.
They should be distinctive enough to make app identification as easy as possible. When I receive a notification, I immediately recognise and respond to the icon. On the home screen, my brain memorises the location of apps on the basis of their icon. You can notice the difference when you relocate an app. You get used to the new location much easier if it's a distinctive icon than if it's a generic one, like a folder. Eventually you get used to a new position, but it happens much quicker with a unique icon. That's the human brain.
Icons should also be descriptive. When I am browsing the App Store, then the icon can often tell a lot about the app. Not just about what it does, but perhaps also whether it is well-designed or not. For new users or users who barely use their devices, descriptive icons can be very helpful. An icon depicting a calendar or envelope can hardly be misunderstood. An icon with coloured bubbles is meaningless (Game Centre).
My problem with the flat style of iOS 7 is that icons are less distinctive and often also less descriptive, or not descriptive at all. Many icons now have two-colour gradients, often the same colours, and a simple, white image. They may look good, but they are less distinctive than their predecessors. Many of them are also not descriptive (anymore). That may be less of a problem with Apple's apps, but third-party developers are following the lead, as users start to experience a certain aversion for everything that is not flat. We end up with lots of simplistic icons and while it may fit some people's tastes, they are less functional than they used to be. For me, that is a step backwards, and even worse, an unnecessary one.