This is also one of my minor complaints about OS X after switching from using Windows all my life. It's so much better functionally to allow the user to resize the window from any side or corner. But, have you noticed that a lot of OS X apps don't even really have anything to grab onto on the window sides? A good example of this is any OS X web browser. Maybe Apple doesn't think it would make sense to "grab" onto a non-existent window side.