When an application (e.g. Firefox), is open and active, clicking on the icon in the unity launcher does nothing. Instead, you must minimize it by clicking on the minimize button in the upper left hand corner of the screen. Currently, clicking on an active open application does nothing.
What SHOULD happen is that when clicked on, the application should be minimized, and show active applications/windows under it, or the desktop, as in GNOME with the panels. This will create a more consistent and familiar experience. Quite an annoyance from my usage.
Please let me know if there is a specific reason for why this does not occur, or if this is something to be added in the future.