Office Web Apps - Can’t View Office Documents in Browser !!!

After installing Office Web Apps 2010 on SharePoint 2010 Server you expect viewing and editing on the browser your office documents.
   
Regarding which Web Application is hosting the Document Library containing this documents you should ensure that the Application Pool Accounts used for :

- Word Viewing Service Application
- Excel Service Application
 
- PowerPoint Service Application
Are allowed to access its content databases.