Eh, I go through phases with MMO's. I've played them off and on for decades, my earliest experience being Ultima Online, followed by EQ, and so on up the MMO evolutionary chain. I find them fun for the most part, but the repetition of actions without any tangible effect on the world around me makes me tire of them really fast. Since it's a persistent world, where they have to allow everyone to have the same content experience, there isn't anything unique about any of your characters. You all just repeat the same actions over and over.
A good example, the game ArcheAge. In the tutorial/opening hours of the game, you come up to a particular stone along the path that the story quest sends you. There is a young woman standing by the stone...apparently because she just likes standing by rocks next to a path. She bubbles and giggles at you while talking ( I really don't understand why, other than she's an anime inspired female character and thus...you know..bubbly), and she tells you to touch the stone. You do so, and a brand burns on your hand, marking you as The Special Snowflake of Legend, Not Seen in Centuries. That's fine and all....eeeeexcept for one little problem.....
The rock is surrounded by about 3 dozen other players, all getting the same speech as you about being Super Special Snowflake.
Thus, the cool factor of this "revelation" is instantly diluted by the fact that everybody's doing the same story, with the same stuff, and there really isn't anything dynamic going on. It's everyone playing the same character in a well read script, without any real impact on what's going on.
Now some games lessen this problem by allowing player controlled/run content, but it's very minimal in my experience, and doesn't impact anything outside of those specific zones where this behavior is allowed.
So when I'm playing a game like that, or any of the MMO's really, my interest starts to tank dramatically as I feel the repetition and grind of it. Which is usually when I stop playing.
But that's one reason why I like the FTP model, because I can easily stop playing some game when I get bored with it, without any worry that I'm wasting money or not. It was fun for a bit, and then it was put aside.