Adaptive difficulty (also called Dynamic Difficulty Adjustment) is a game mechanic in which play becomes easier or more difficult depending on how well you progress. If you keep dying in the course of the game, the action ramps down a bit until it's at a more manageable level. This can be seen in the God of War and Devil May Cry franchises. As an uneasy compromise between the hardcore and casual players, it serves to make things easier initially, but at the cost of preventing access to content unlockable only to those willing to put in the time to master the challenges, as difficult as they may be.
But in an MMO, where the gameplay is meant to progress conceivably into eternity, does adaptive difficulty help or hurt the player? Should the challenge remain static, and act as a determinant of inclusion, a kind of 'survival of the fittest'? If gameplay is accessible to any level of skill, should a monthly fee be in place to take advantage of the hordes of relatively non-hardcore users?