How to choose Gnome instead of the new default desktop environment (Unity) in Ubuntu 11.04 (Natty Narwhal)

See here for more explanation.

It's surprising that in pre-release versions of Ubuntu 11.04, the choice of desktop environment could be made from the login screen, but after release the choice must be made in this less-obvious manner. Let's try to make it easier to choose among the alternative desktop environments, not more obscure.

In Ubuntu 11.04 with Compiz enabled, the System->Preferences->Windows, Titlebar Action "Maximize Vertically" doesn't work any more, it behaves the same as "Maximize". Being able to maximize vertically is very convenient, especially for terminal users, it's surprising this hasn't been fixed sooner. You can work around the problem using this idea, by assigning a keyboard shortcut using the gnome-keybinding-properties program (by navigating to its Window Management subsection to find the Maximize Window Vertically action and assigning a keyboard shortcut for it). You can also maximize vertically by middle-clicking on the little square "Maximize" icon in the window's title bar (found this by reading the Unity Cheat Sheet (mirrored here) and this page). Apparently the middle-click on the Maximize icon is a feature of the Compiz window manager, see here for explanation.