Despite all the work on dual factor authentication and other new security methodologies, in general our passwords are the keys to the kingdom.
In many cases, such at ATMs, we are limited to 4 digit numeric PINs.
This post to DataGenetics does a good job of analyzing how bad we are at picking PINs and how easy we make things for the attackers.
It is worth a read.
Short answer: you can hack a over 10% of accounts by guessing "1234".