Comment 1 for bug 791205

Update:

The scope is in fact bigger than previously thought: 2 other options were forgotten that need to be set only in a Unity 2D session:

/apps/metacity/general/show_maximized_titlebars false
/apps/metacity/general/auto_maximize_windows true

A new simpler, more generic solution was thought by Didier: adding a command line option to metacity where one could pass a file containing a set of GConf key/value pairs that override the current ones.