You can also have action buttons in notifications. See, for example, http://minus.com/lbpCsG which is a minimal MPD client written as a notification for gnome-shell. In gnome 3.6, you can interact with it using only the keyboard (<Super>M, left/right to select the notification, up, left/right to select the button and then enter to activate it), which you can't do with the usual status icons unless the app is programmed to behave like that, and which I think you can't do either with indicators like those used in Unity (though I haven't used that in a while, so I might be wrong).