How to change password rules on Raspbian

On recent version of Raspbian (I think since the release of January 7th of 2014) the password rules have changed and you can’t use anymore simple password like `pi`.
To enforce this requirement, just change the line 25 of /etc/pam.d/common-password. Remove the obscur keyword and add minlen=2 (or whatever you want).
The line should looks like :
25 password [success=1 default=ignore] sha512 minlen=2

Check man pam_unix for more options.