Difficulty of normal content.

[Jaxtaro]

There are two types of MMO players:

People who think the gameplay is fun and the rewards are an incentive to do the things they wanted to do anyway.
People who think getting rewards is fun and the gameplay is the cost of getting them.

This isn't a value judgment; I'm personally in group 1, but I know plenty of bad players in group 1 and plenty of great players in group 2, and I have decades-long friendships with people on both sides of this divide.

Still, it has a huge impact on which content you enjoy and why you enjoy it.

I mean, these are definitely two prominent categories, but there are a lot more than these two categories. It's the reason behind the mass appeal of MMO's.