What's troubling is that apparently there was evil code running on twitch servers, capturing plaintext passwords as they were checked against the "cryptographically protected form". [1]

[1] http://gyazo.com/b31792d4fc4460c006c49d858cc113e3

Here is the email sent to users:

We are writing to let you know that there may have been unauthorized access to some of your Twitch user account information, including possibly your Twitch username and associated email address, your password (which was cryptographically protected), the last IP address you logged in from, and any of the following if you provided it to us: first and last name, phone number, address, and date of birth.

For your protection, we have expired your password and stream keys. In addition, if you had connected your account to Twitter or YouTube, we have terminated this connection.

You will be prompted to create a new password the next time you attempt to log into your Twitch account. If applicable, you will also need to re-connect your account to Twitter and YouTube, and re-authenticate through Facebook, once you change your password. We also recommend that you change your password at any other website where you use the same or a similar password.

We apologize for this inconvenience.

The Twitch Team

here's a screenshot of the email sent to an 'affected party'

http://gyazo.com/b31792d4fc4460c006c49d858cc113e3

For partners, their personal details being leaked is much worse. Think more swattings, pranks etc.

TL;DR log on to twitch and reset your password

they haven’t expired sessions which has worried me a little.