They reworked the whole new-player-expierence, plus traits for a better character progression. Same reason why Mastery System is being introduced.
They revisited old areas, adding events and challanges, adding new bosses(3headed wurm, marionette) and reworking bosses (tequatl and the fractal ones) in ways we havent played them before.
I will try to explain myself with an example.
For the sake of the example, lets say HoT will be released in the end of May.
So, with that in mind, when HoT releases in May, we will have all the new features and stuff to play with and everyone are happy that GW2 finaly got an expansion.
But, if (big IF) at PAX Anet announced Living Story Season 3 along side new and long awaited features, and said the new episodes start rolling out from Febuary to the end of May. It would go like this - the new zones of the Maguuma Jungle will be released with the episodes (much like karkaland, dry top and silverwastes), the story missions(and with the return of Rytlock - Revenant will be introduced) and challanges (and their coresponding jungle masteries) would unfold as the episodes roll out. In between the episodes they would have a feature pack adding guild halls and the core game masteries. And at the end of the season, another feature pack will introduce the new sPvP game mode and the WvW borderland and reworked fundamentals.
In the end we would have the exact same content, just rolled out bit by bit (as it was until now), but most of reddit would continue to bitch, why this game hasn't got an proper expansion.
Hope this clears what I mean.