ArenaNet has made it mandatory for all Guild Wars 2 players with accounts created before September 12th 2012 to change their passwords from Thursday this week (7th Feb), the company has announced in a new blog post on the Guild Wars 2 website. Players who have signed up for Guild Wars 2 accounts since the 12th of September will have already been forced into choosing more secure passwords after the introduction of ArenaNet's “blacklist of passwords that have been used in the past by hackers to scan for accounts”, but those who made their accounts before that was introduced will be required to change their passwords to more secure ones from Thursday onwards.
These players will actually have seen a red banner in the game's launcher for the past three months encouraging them to change their passwords, so many will have done so already. But those who haven't, for whatever reason, will have no choice come Thursday. If you're particularly attached to your password, you'll have to abandon it, because the system won't allow you to pick your previous password or one that ArenaNet has “seen tested against any existing or non-existent account”. The bad news is you'll have to do a bit of thinking to come up with something new, but the good news is once you do you should feel a little more secure.
ArenaNet suggests checking out the XKCD comic strip on passwords for inspiration.