- Their passwords aren't case sensitive.
- Their login form's GET request isn't using https.
- Within 20 minutes of informing a security specialist not affiliated to blizzard he was able to write a script that compromised every single battle.net e-mail address.
* This information is based from about June 2012, I'm not sure if their security measures changed since then but I remember the thread on the bnet forums and it being confirmed by an official moderator-like person saying blizzard was aware of the problem for the longest time but didn't care about fixing it.
I never understood how game companies could be so lazy when it comes to security. They obviously have the talent to do things right and rake in silly amounts of $. Why jeopardize the integrity of their name.