Desktop environments (Unity, GNOME 3 and no classic GNOME 2) in Ubuntu >= 11.10

In Ubuntu 11.04, it was possible by default to use Unity (which itself run on top of GNOME 2 core) or classic GNOME 2.32.1.

Unfortunately, Ubuntu 11.10 (and 12.04 LTS) comes with Unity shell as default desktop environment and doesn't have a classic GNOME option; instead of it, there's a Unity 2D option (in Ubuntu 11.10, Unity runs on top of GNOME 3.2 core components).

GNOME 3 itself is completely redesigned and doesn't look like "classic" GNOME 2; IMHO, it's not any better than Unity in functionality.

Linus Torvalds switched himself to Xfce because of GNOME 3 "improvements".
Eric Raymond (who had used Red Hat and Fedora in the past and switched to Ubuntu in 2007) strongly criticizes both GNOME 3 and Unity and plans to switch to Xfce too.

If you wish, you can try GNOME 3 Shell environment by installing it:

$ sudo apt-get install gnome-shell

At last, there is a "classic" fallback mode in GNOME 3; it also can be installed manually:

$ sudo apt-get install gnome-session-fallback

After that, you can log in selecting "GNOME Classic (no effects)" (or "GNOME Shell", or "Unity") at the login screen (I don't recommend you to choose "GNOME Classic" as it uses Compiz, in the buggy way).

One note about GNOME 3 "classic" fallback mode: to call a panel context menu, you should right-click on it holding the <Alt> key. This way you can add some "classic" applets (e. g., "Show Desktop" and "Workspace Switcher") and application launchers to top and bottom panels.
Indicator Applet is missing in Ubuntu 11.10; fortunately, it was ported to GNOME 3 and comes back in Precise (12.04 LTS); if you want it in Oneiric (11.10), you can follow that article: Indicator Applet Ported To GNOME 3

If you don't like Unity, you can (almost) painlessly uninstall it. Don't forget about removing some useless components (they are installed with Unity and continue to run even in your plain GNOME session) like:

• overlay scrollbar (scrollbar overlayed widget);
• BAMF (some "application matching service" used in Unity?);
• Zeitgeist (background user actions logger and indexer).

If you don't like the default Ubuntu 11.10 login screen, just install another greeter:

$ sudo apt-get install lightdm-gtk-greeter

You can even remove LightDM and install the updated GDM 3 display manager (used in GNOME 3 by default).

If you don't have a Synaptic GUI package manager which is useful for package searching, installing and removing (IMHO, Ubuntu Software Center is too weak), you can install it manually:

$ sudo apt-get install synaptic

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Updated (07.04.2012) to reflect Ubuntu 12.04 changes etc.