The username-less login is a really useful UI improvement – now, after each login on the website, the page stores user mail hash and usernames in the cookie file - this information is used to create the following block on the next visit at the login page:
As you can see in this case, a user does not have to remember the login – he/she has to remember only his/her own password. Of course if the other user uses the same computer, then he/she can click "Not Super User?" link - then the form will back to the original layout:
The Username-less login can be enabled in the Advanced settings tab at the template settings.