Aside from the background shading mentioned here in the comments, the major difference between Quick Flag and Flag Status is the ability to flag an item with just one click. By left-clicking on the Quick Flag, it will set that message to the default color (red) follow-up flag. You can't do this with the Flag Status; you have to Right-Click>Follow-Up>Red Flag. At least, that's the way it works on Outlook 2003.