D&D has always been focused around fighters, clerics, rogues/thieves, and wizards. The main systemic difference role-wise was that the designers recognized that you need to fill these roles, and making sure that other classes could do so as well, instead of falling in between the roles and breaking the Ron Swanson rule by half-assing two things. As part of this system change, Defender classes were given the ability to manage "aggro", albeit via punisher mechanics ("You're free to do X, but I'll hurt you if you do") instead of aggro lists like in WoW.
There was also a more specific change with the fighter and ranger, where the ranger became more purely martial and took most of the heavy damage/Dex-based/archery stuff from the fighter, and the fighter became more defined as specifically a Defender.