A while ago I talked about games of advancement, a wider term for games which have the player gain levels, skills, or stats over time. These are everywhere these days, and even when people discuss "gamification" of the real world they usually mean turning some aspect of the real world into a game of advancement. Advancement has been shown to be a very strong motivator, at least in the short term. But what about the long term? In the long term games of advancement very much suffer from problems with burnout.
The source of this burnout problem is one of limited resources: No game has infinite content unlockable through advancement. Being very well aware of that, developers design advancement with built-in diminishing returns: The further you advance in the game, the slower progress gets. The first levels are always gained the fastest, and then every further level takes a bit longer to reach. Or there are level caps, where further advancement is only possible by much slower methods, like gathering equipment.
At the same time the novelty value of the game constantly diminishes. Or as Raph Koster would say, the learning experience leading to fun in a game diminishes the more you already learned. Unless you get holy grail of gaming, the "easy to learn, hard to master" game, you learn less and less with each session.
The advancement part of games is designed to prolong the learning experience, by giving you access to new content to learn about over time. When there is really nothing left to learn about, developers hope you'll still be playing for the advancement. But with that advancement getting slower and slower, at some point the slow pace of advancement isn't fun enough to compensate for the diminished fun of the activity. At that point players start talking about "the grind", and from there to burnout it is just a short way.
I do think that the diminishing return design of games of advancement is a mistake. Rushing players through the early levels isn't all that helpful, and the lure of advancement stops working in the late levels when progress gets too slow. A system in which every level takes about the same amount of time would probably work better for long-time motivation. And both players and developers have to realize that no game is fun forever, so there is actually nothing wrong about a game over screen at the end. There are a lot of games where starting over is more fun than to keep playing at the highest power level. From the MMORPGs I know, only A Tale in the Desert does this, although there is also a reset rumored to be part of a "re-release" of Darkfall "2.0". Some Japanese single-player RPGs have systems where finishing the game once gives you new options for playing the game a second time. Maybe such an approach would work better for MMORPGs than letting player repeat the same dungeons and daily quests for months.