If you miss the extensions and themes of Gnome 2, I thought I'd point you to a few Gnome Shell extensions that are available to you. These are the package names, and a description of what they do:
gnome-shell-extension-cpu-temperatureSimplify your desktop
Adds an applet on the panel which displays the temperature of your CPU.
Allows you to add and remove icons from the top bar panel by simply editing your gsettings.
Displays player controls, music, and music cover in the Gnome Shell.
Makes it easier to use the keyboard to switch windows and workspaces. When you're in "overview" mode ("show all windows" mode) press the Alt key to show numbers in each window. Just press the number for the window you want to display.
Adds a "workspaces" menu to your status area, to make it easier to switch between spaces.
Changes the behavior of Gnome Shell back to "classic" Alt-Tab behavior. In the default Gnome 3, Alt-Tab will switch between applications, but separate windows for each application are grouped together. This extension disables grouping, so that you switch between windows rather than applications.
Adds "Suspend" and "Power Off" options to your status menu. In the default Gnome 3, you have to press Alt in the status menu for "Suspend" to become "Power Off".
Lets you manage your workspaces by assigning a specific space to an application as soon as it creates a window.
Puts a task-switcher "dock" on the right side of your screen.
Adds a menu in your status area to show removable media.
Lays out the "thumbnails" in the window overview that reflects the positions and relative sizes of the actual windows.
Adds a menu in the system status area that resembles the Places menu from Gnome 2.
Removes your name and IM status options.
Displays the "ripples" that show up when you enter the "Activities" hot corner.
Integrates Pidgin chat into your Gnome Shell session.
Removes the "Accessibility" menu from the top panel.
Removes the Bluetooth icon from the top panel.
Removes the volume icon from the top menu.
Adds a timer to help you track time, if you use the Pomodoro Technique for time management. (Essentially, break work units into 25-minute periods, and work exclusively on that task for 25 minutes.)
Gnome Shell user theme selector, with preview.
Lets you select a custom theme for the Gnome Shell (from ~/.themes/theme/gnome-shell/gnome-shell.css)