Massively -- aren't at all unified when it comes to what we actually accept as "content." Some players don't care if a given piece of content plays exactly like another, so long as it's visually new and interesting and different. Others want a distinct set of lore behind every new piece of content, with the visuals and gameplay coming far later. And still others will complain if the additions play the same, believing everything else secondary to new mechanics.
In short, all developers need to do is constantly churn out a stream of new content with unique textures and models, unique mechanics, and unique lore, and everyone will be happy. Well, except for players who want to see expansion to the game's underlying elements... you get the idea. So what do you want, first and foremost, in your new content? What elements are most important to convince you that it's new and improved, and what elements just seem to be a cheap recycling of existing assets?