This is a mockup. It might never happen, and it more likely will be different when it comes. Popover is a new widget for Gtk and I think we are going to see them a lot in the future, in Gnome 3.14 in September 2014.
No need to explain the differences between a Popover and a classical menu, but here is the outcome.
That’s Totem 3.11.x under development, and Gear Menu looks really bad, with a two level navigation. To be honest Nautilus Gear Menu isn’t either much better. What if we add a popover here?
One thing with Popover items is that they cant’t have children(submenus). If you add children in Popover you will just get an accordion menu, coming straight from 80ies ;)
Gnome Shell makes some soft use of children items in popovers, but doesn’t really abuse it. As a rule, popovers can display more complex context than menus (like Shell Calendar), but they can’t list many items, except if you are going to create mega menus (multiple lists), which isn’t a common technique in desktop.
That’s the mockup of Totem that Jakub Steiner (Gnome Designer) uploaded today. Notice the 3 dots in items, that indicates that something else is going to happen. Since there are dots and not an arrow, I guess this won’t bring a slide-bottom submenu.
I think this is a very nice improvement, that it will affect many gnome-modules.