While I like many of the ideas in Gnome Shell, juggling between multiple applications can be unwieldly. For example, I often find myself programming with a text editor, a terminal, and a browser open. It's frustrating in Gnome Shell.
The following workflow mockup is my attempt at fixing the problem without compromising the design imperatives of Gnome Shell.
The user has two activities he wants to work in close conjunction.
The user zooms out. He has three activities open.
The user drags one activity onto another.
The two activities join into one group activity, bound together until the user drags one apart.
The user is working with another activity. He alt-tabs, and finds group activities visually associated.
A group activity is selected, and the sub-activities appear tiled horizontally on the screen. Note that whichever window was specifically selected will have focus.