This article is about the classic GNOME-2-like desktop. It is neither the Unity desktop used in Ubuntu nor the GNOME 3 "GNOME Shell" desktop.
Ubuntu 11.10 (Oneiric Ocelot) and later
In Ubuntu 11.10 (Oneiric Ocelot) and later, GNOME 2 has been replaced by GNOME 3. The default desktop is Unity, with Unity 2D as a fallback. The default GNOME 3 desktop, GNOME Shell, is available via the The gnome-shell package. However, GNOME Shell is controversial because it is also weird and not very customizable. A "classic" GNOME desktop option is not shown by default. However, a "Classic" desktop built upon GNOME 3 is still available.
Install the gnome-session-fallback package (from the universe repository), log out, and choose GNOME Classic at the login screen.
The only remaining supported Ubuntu release that supports the gnome-session-fallback package is Ubuntu 12.04 (Precise Pangolin). It is not available in newer releases.
One the most visible changes is the lack of a System menu. The essential functions of this menu have been moved to the user menu on the right side of the panel. Notably absent from the user menu are the Preferences and Administration submenus. In their place is GNOME 3's System Settings.
In Ubuntu 11.10, any remaining configuration panels are currently in Applications → Other.
In Ubuntu 12.04 and later, the Preferences and Administration submenus have been moved under Applications → System Tools.
Applets & Panel Layout
Panel applets can be modified by holding the Alt key when you right-click on the panel. Applets are, however, no longer freely positionable. Instead, applets may be anchored to the left, center, or right of each panel. This new scheme allows the panel's layout to persist across screen resolution changes.
Ubuntu's version of System Settings contains a basic theme selector in the Appearance panel and a coarse text size setting in Universal Access.
GNOME Tweak Tool
You can customize themes, fonts, and various other hidden settings by installing gnome-tweak-tool. Once installed, it can be found at Applications → Other → Advanced Settings. Note that you will not be able to use your preferred GTK2 theme unless it has been ported to GTK3's new theme system.