I think it's the backgrounds everywhere that make iOS 7 look so busy, especially the home screen. The screens that are simple text-on-background or boxes-on-background look fine, and the chatbox retains a pure simple white background.

In cases where the white background exists, the stylistic similarities to WP7/8 look more obvious. WP7/8 make very limited use of elaborate backgrounds, preferring to go with jet-black. There's an option to invert the WP7 colors and go with black text on white background... it makes iOS7 look like a complete clone there.