Directory.io first of all does not contain all private keys - it's more of a joke.

Anyway, if a brain wallet has a weak password, you have quite a good chance of cracking it easily. But you have to know that it's a brain wallet. But using a brain wallet is just silly.

Also, don't forget cracking private keys using weak signatures, although good luck finding someone who has a wallet and a weak signature...

http://www.nilsschneider.net/2013/01/28/recovering-bitcoin-p...

EDITED - I didn't read the post correctly, my apologies.