Minimum Functionality: while there is no clear specification as to what is minimum functionality, try adding more native functions (things you cannot do on a website, functionalities that can only be used on mobiles) into your app to make it more "app-like" if you get rejected for this reason.
In case your app is rejected due to "Guideline 5.1.1 - Legal - Privacy - Data Collection and Storage", ensure that you provide account-specific functionality when you require user login before using the app.
Apple does not allow selling digital goods with out using its in-app purchase platform.
You cannot change the functionality of the app after it has been reviewed. This will also be a cause for rejection.