The biggest change is that Apple will begin to remove outdated apps from the App Store that no longer comply with the new current guidelines or haven't been updated to support new APIs or features. Removing old apps will also reduce the number of apps that haven't been updated for compatibility with newer iOS versions or iDevices.
Apple isn't saying exactly what criteria it will use to remove apps, just that it is "implementing an ongoing process of evaluating apps for these issues, notifying their developers, and removing problematic and abandoned apps." People who have already downloaded these apps will still be able to download them, developers will be given 30 days to issue an update before their app is removed, and if their apps are removed they will still be associated with that developer's account and other developers won't be able to use the names for other apps.
The company is also ready to start the fight against spammy app names. For instance, if you search for “Instagram” on the App Store, one of the first results is an app that is called “Photo Collage, Picture Editor” These apps will get cut off.
Apps that crash on launch are also not left out, They will be removed immediately.
Another change is related to app names. Apps submitted to the App Store will now have their names capped at 50 characters. This is meant to cut down on the number of apps that are having long names with multiple keywords intended to improve search ranking.
Apple will start reviewing old apps on September 7. Below, see one of Apple's official messaging as regards this change.