Via Marco Arment:
Combined with @lukew’s suggestion of pre-populating the username field, this design would allow existing users to login with only a few clicks and no typing — select an account from a list, click a link, and they’re done. For users who do not already have an account, the process is the same - they will type only their email address, and receive a link via email. This email logs them in and validates the email in one fell swoop.
The web as currently ‘designed’ puts all the weight of security on its weakest link: users. Users are never going to get any better at selecting or remembering good passwords, so web designers should try as much as possible to remove this burden from the shoulders of users.