The user object just got two new attributes, id and active_supporter. The id is a unique identifier that unlike the username, can't be changed. Although it can't be used as an identifier in API calls yet, you should use it whenever your application needs a unique identifier.
The second new attribute is active_supporter, which tells you if the user can flattr other users or not. Until now the only way to tell was to to actually flattr something and look at the response, an error meant the user couldn't. I think you will agree that this way is cleaner.