Apple is expecting iOS 7.0 to be downloaded by "hundreds of millions of people". In terms of compatibility, keep in mind that not all the new features will be available across the iOS devices. You can learn which devices will get which features by checking here.
Apple says that iOS 7.0 contains "over 200 features" including the completely revamped UI, the new share menu, AirDrop, new camera features, new "textured ringtones", better multitasking, better battery management, and Siri improvements.
source - Apple