The biggest change was an overhaul of our authentication system. Essentially the login system hasn't changed since Harvey wrote it 10 years ago (!!), and the hash algorithm used at the time only read the first 8 characters of your password. So until this morning, you could omit anything after the first 8 characters of your password when logging in.
Todays update changes this. Now passwords are stored in an ultra secure bcrypt
-based "slow hash" algorithm. Now you have to type in your full password to login.
The update also included a few bug fixes and an update to the Quick Edit tool in the media galleries.
You can always find out what we changed by reading the "Version Info" page linked in the footer.