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] pam_unix.so sha512 minlen=2

Check man pam_unix for more options.