Tibbles are not just data frames with extra class attribute. For one - they don't have row names. Second, consider this example, demonstrating how treating tibbles as data frames can be dangerous:
Ok, fine, to be more precise, tibbles and data frames and data tables are all implemented as R lists whose elements are vectors which form the columns of the table. And also `is.data.frame` currently returns TRUE for all of them, whether or not that is ultimately correct.
