My computer display is not big so I have tried ways to save my screen real-estate. It looks to me that the designers of GNOME 3 must have a huge display.  The default window title bar are too high in my view. Besides the top panel is thicker than that in Unity. To make my life easier, I first installed the GTK+ 3.x theme from gnome-look.org to reduce the height of the title bar. Then I set the top panel of GNOME desktop to 'auto hide'. But whenever I want to look at the weather or the time I have to move mouse over it to make it show. This is tedious. I searched online for a shortcut to show it. Many of results are prior to GNOME 3. Besides, I don't like the methods to change the 'gsetting' or 'gconf'.

It turns out that the simplest way to to hit 'Alt+F1' to activate the 'Application' in the top panel and hit 'Esc' to hide it again. So simple.

BTW: I'm using the fallback of GNOME 3.