XKCD #936: Short complex password, or long dictionary passphrase?

I think the most important part of this comic (XKCD #936), even if it were to get the math wrong (which it didn't), is visually emphasizing that there are two equally important aspects to selecting a strong password (or actually, a password policy, in general):

• Difficulty to guess
  
• Difficulty to remember
  
• Or, in other words:
  
• The computer aspect
  
• The human aspect
  

All too often, when discussing complex passwords, strong policies, expiration, etc (and, to generalize - all security), we tend to focus overly much on the computer aspects, and skip over the human aspects.

Especially when it comes to passwords, (and double especially for average users), the human aspect should often be the overriding concern.

For example, how often does strict password complexity policy enforced by IT (such as the one shown in the XKCD #936), result in the user writing down his password, and taping it to his screen? That is a direct result of focusing too much on the computer aspect, at the expense of the human aspect.

And I think that is the core message from the sage of XKCD - yes, Easy to Guess is bad, but Hard to Remember is equally so.

And that principle is a correct one. We should remember this more often, AKA AviD's Rule of Usability:

Security at the expense of usability comes at the expense of security.