The second is the overview, which contains the menus and views of the workspaces you define. On the left of the overview is the menu, which from top to bottom offers an application search field, a list of all applications, a list of favorite (and open) applications, links to places and devices, and, at the bottom, recently opened applications.
On the right of the overview is the workspace display. It is a horizontally-scrolling display of workspaces, with a link to each one below.
The constant between the workspaces and the overview is the panel, which from left to right includes Activities (a link to the overview), the currently active window, A clock and calendar, notification tray icons, and a menu from which you can centrally declare your status for chat and social networking, or log in or out. So far, there are no applets that can be added to the panel.
If you are already a frequent user of multiple workspaces, then you will immediately appreciate the Activities overview. Not only does it offer easy switching back and forth between workspaces, but it offers a thumbnail in which all the applications running on each workspace are clearly visible. Its workspace display is much cleaner than the cramped horizontal scroll currently offered in KDE (or, for that matter, than the Zoom Out view that the early KDE 4 series offered).
Similarly, if you are jealous of every square centimeter of screen display, you might appreciate the exiling of menus from the workspace. Even if you do not use what KDE calls a classical menu -- one whose sub-menus cascade across the desktop -- you no longer have to worry about obscuring part of the desktop with the menu.
However, if you do not use multiple workspaces, or use a menu frequently, you may very well become annoyed by the constant requirement to shift back and forth between your workspace and the overview.
The only other alternative is to set up one workspace with all the icons you are likely to need, so that you rarely need the overview. But the main tendency in GNOME Shell is to pressure you into using the overview, whether you want to or not. Even to see a minimized application requires you to switch between the workspace and the overview, unless you happen to remember what workspace the application is on, and the workspace is not too crowded.
To further the pressure, the overview of a workspace, with thumbnails neatly arranged in rows, is not what you find when you zoom in on a workspace. Instead, what you find on a workspace is a stack of open windows, and no easy way to search through them. Instead, what you have to do is select a window from the overview, so that it is active when you zoom in -- which gives yet another reason to switch constantly between activities and overview.
Probably, too, you should not open too many windows on one workspace. But that means opening more workspaces, and increasing your need to be constantly switching views.
No matter how you look at GNOME Shell, the result is the same: either you are going to do a lot of clicking, or else you will need to learn the keyboard shortcuts. Either way, the momentary pause as the screen redraws may be a distraction to your thoughts.
The lack of flexibility in the general design is reinforced by the menu on the overview, at least in its present state. Currently, at least it is uneditable, and unscrollable, too. If you prefer a file manager to a series of links, or want links to other locations, you are out of luck.