One recurring complaint of MMO players is the "grind". It's interesting taking a look at the history of MMOs, because it becomes obvious that it's mostly player perception that determines what is a grind. Every mechanic that people deride as a grind started out as something fun. In fact, sometimes it is a particularly new or novel feature that eventually becomes the dreaded grind.
Let's take a look at how something fun devolves into a grind.