Another factor to consider is, as a whole, MUSH-dom doesn't do subtlety very well. When it's there to be found in RP it isn't usually so much an undertone of racism or discrimination as much as a consistent stream of wide-eyed ranting and gleeful mustache-twirling delivering monologue after monologue about whatever the character is about.
Movies and books can actually portray this kind of villainy to add some depth and set a tone without making these aspects the every scene's sole focus. PCs often don't. Far more so when their players are disingenuous about their characters' purpose in the game.