They're aiming at making sure that you no longer will outlevel your zone and be forced to move somewhere else every few levels, which means that if you want to actually follow a questline and complete it, the game will no longer discourage you from doing so.
Levels 60-80 will allow you to quest in Outland or Northrend interchangeably, allowing you to experience those quests in whatever order you desire. That means you can stay in your favourite zone for as long as you wish, whether it is Icecrown or Hellfire Peninsula.