Security / Privacy disadvantage
There are security and privacy risks involved with this approach when badly implemented. An attacker could figure out if the email- or login already exists, when not having a default flow. As mentioned @Mario Trucco this can also be done via the registration process.
- Security is at risk: Because it becomes easier for an attacker to bruteforce their way into a system. It will make guessing easier if you only need to guess a password, instead of both the username and password.
- The privacy of users is under scrutiny. (Other people will know if you are listed at that website.)
Reasons why this is implemented
I found this on the Google documentation:
This new Google account sign-in flow will provide the following advantages:
- Preparation for future authentication solutions that complement
- A better experience for SAML SSO users, such as university students or corporate users that sign in with a different identity provider than Google
- It may enable more personalized customization options for security such as phrases or images providing more security options (see example below). This would reduce the scope of phishing as the screen generated would be specific to the user and would vary from user to user.
- Because users can have different ways of authenticating and the identity of the user is equal to the username this will make it easier server-side to redirect traffic to the users form of authenticating
-v The users sees 'his/her' personal Image or Sound. If that image does not correspond with the image given at registration. The users knows this is a fake login.