Quick update: The Unity 7 minimize on click feature has landed in Ubuntu 14.04 Trusty Tahr.
Minimize on click is not enabled by default and to enable it, you need to use CCSM:
sudo apt-get install compizconfig-settings-manager
And then, open CompizConfig Settings Manager, click the Ubuntu Unity Plugin and on the Launcher tab, you'll find an option called "Minimize Single Window Applications (Unsupported)".
Later on, this feature will most probably be configurable through Unity Tweak Tool and other such applications.
Like the option name says, the Unity Launcher supports minimize on click for single window applications only so if there are 2 open windows for an application, minimize on click won't work.