Thus we can expect leveling from levels 60-80 to take relatively as much time as it takes to level them at the moment.
More about the cause to the Cataclysm:
To keep Azeroth safe from these threats, the titans imprisoned the Old Gods underground and banished the savage elementals to the titan-constructed Elemental Plane, a secure dominion where the elementals could dwell without harming Azeroth.More about outland and Northrend:
Four domains were created within this primordial realm to serve as ideal environments for each type of elemental. For ages since, the elementals of air, water, earth, and fire have engaged in endless strife with each other, waging titanic battles in the confines of their realm. One of the Elemental Plane's domains, a vast ocean known as the Abyssal Maw, is home to the water elementals. Legend says that Neptulon the Tidehunter, the powerful being who rules over the aquatic realm, possesses a potent weapon that allows its owner to control the very seas of Azeroth. Thus far, the artifact has remained secure inside the water elementals' domain... but that will soon change.
Within Deepholm, the domain of the earth elementals, the malefic Dragon Aspect Deathwing is on the verge of executing a catastrophic act: to burst forth from the Elemental Plane into Azeroth. Doing so will ravage the surface of the world and breach the barrier that separates Azeroth from the elementals' realm. Whether Neptulon and his subjects have entered into an alliance with Deathwing remains unknown. Regardless, if Deathwing's actions create an opening into the Abyssal Maw, nefarious and honorable factions alike will seek out the fabled weapon carried by Neptulon. Whoever secures the artifact will have control over the world's seas, a power that perhaps no one, not even members of the Horde or the Alliance, should possess.
Just because Azeroth is being affected doesn't mean the threat of the Burning Legion isn't still there (for new toons to experience at least). If anything changes in this regard it would probably be a couple quests that send you there but I wouldn't expect Outland itself to change much since messing with 2 continents is already a lot of work. (Source) [...] You have to consider the changes with some form of leniency, while technically you could say that if the story progresses then we need to remove a bunch of the content from Outland and Northrend as those are over - but for numerous reasons, you can probably see why that would be a bad idea. This is the same type of thing that happens by just having a persistent world, just because one quest was completed where something was killed doesn't mean it vanishes from your sight forever. While a level 80 doesn't need to return to the locations of Outland and Northrend and can focus on the new stories - a new character hasn't helped defeat the villains in those areas so they still need to answer the call and fight them off (or in the terms people are tossing around, they need to level)..