I think this is a very important question to ask and there seem to be two schools of thought:
1. Challenging content keeps people playing
Reasoning: Once you've killed the final boss and overcome all challenges there are ingame, you've essentially "beat the game". There's no logical reason to continue playing.
Pro:
-Overcoming a challenge in order to face the next, more challenging, challenge is a satisfying and acknowledged form of progression.
-There's a sense of accomplishment with every challenge overcome.
-There's stuff to look forward to.
Con:
-Content is somewhat "gated" and only becomes accessible after overcoming previous challenges (progressing from normal to heroic).
-Easy content is limited
-Less and fewer players get to participate in each tier of challenges, as the increasing difficulty filters out lacking players.
2. Easy content keeps people playing
Reasoning: You have a wide range of accessible content available to you. More variety in game-play allows for longer-lasting motivation.
Pro:
-Everyone gets to experience a wide variety of content. There's no "gating".
Con:
-A diminished sense of character progression as there are fewer challenges to overcome and thus, less recognition for overcoming said challenges.
-Players who like challenges see little point in playing.
I think both schools of thought follow a certain logic and can be reasonably argued with. I wonder what you think is true.