"Interface Style" Theme Option
A good visual style for widgets (buttons, menus, scroll bars... etc) is subjective.
Some prefer really modern looks with edged and flat UI elements, others like rounded elements with shadows and highlights, others again prefer the old and classic look, ... this may come down to personal taste, but there are practical aspects to it as well.
The ability to select different styles allows introducing new styles without causing frustration to existing users who prefer to keep the interface unchanged.
A possible solution for this is to provide an option for the user to tweak the visual style for his personal preference. More specific this would be a drop down in the User Preferences looking like this:(could contain more items of course)
The idea of the "Interface Style" option is to provide a way for the user to easily change the look and feel of the UI to fit his taste. Recreating other application's or OS' GUIs is out of scope, instead it should give variations that change the visual appearance on the whole.
This was in fact supported in Blender 2.4x, - Example of rounded and minimal style: http://download.blender.org/ftp/ideasman42/pics/blender_24_rounded_minimal.png//