I am also searching for this, its quite annoying when at the end of the day I notice I have loads of apps still running in the dock (the little light under them) and I have to manually click and quit each one! I am getting used to Command+Q but old habbits die hard, its so easy to just click X and think its gone.
This is only true if you use the apps regularly. Its the opposite if you only use it once/rarely then you have an unused app using resources running in the background, which may all build up with many apps over time if you leave the computer on like I do. So it adds to my time to manually close apps all the time.
Edit:
Just found this post http://www.mac-forums.com/forums/switcher-hangout/135405-little-red-button-vs-quit-application.html that mentioned the Command + Tab shortcut to view running apps, combined with a Command +Q to close I think it will be fast enough. It is just getting used to the Command + Q option rather than X I think is the issue.