Our OAuth2 implementation avoids a lot of the bad parts of OAuth2 by simply not supporting them, but it was determined that due to the real monetary value of accounts ArenaNet isn’t comfortable with the core phishing problems that affect any system like OAuth.
Auth is still a possible, in that you can still verify that a user has a valid GW2 account by having them create an API key and enter it. It’s just significantly more annoying. A password manager that can autofill would be the best user experience possible in this new authentication scheme.