Flat style should have been the default when you came to the new site unless you changed it in your preferences. We did not want anybody being confused so we made flat everyones default so it would look and function just like the old site. The tree style was sort of an "opt-in" for those who like that better (and there are many who do - I am not one of them but have spoke to some who love it. I find it more difficult but that is why it is a user option.)