Every developer of Windows Phone 7 applications must know and follow the application certification requirements or their app won’t make it to the Windows Phone Marketplace.
Here is a post that summarizes the 22 page document to approximately 2 pages:
http://www.shazaml.com/archives/summary-of-windows-phone-7-application-certification-requirements
It is meant as a quick reference for the major points and not a replacement for the official document.