Apple Inc. has always been the leader in graphical user interfaces. Apple have always know very well how to create intuitive user interfaces. Apple has always know the difference between a document and an application and has centered the user interface around the document while Microsoft has done the same around the unintuitive and computer specific software application.
Users should not need to know about applications (they do not need to know they have to open the spreadsheet application to create a spreadsheet document) they just need to know about documents or objects and actions (they just need to know that they can click somewhere to create a new document and just clicking on a document it is opened for viewing, editing or printing). Applications should be hidden totally from the user.
I really hate applications that use multiple document interface mostly because they force the user to understand what an application is, instead of just focusing on the object (text document, spreadsheet, image, video, song, ...). It seems Microsoft Office 2007 has finally done some progress to stay away of MDI but Excel still has an internal close button (x).