Ah yes. That's very true - the typical Mary Sue tends to preach a lot and improve everybody's life. She silkyfies Snape's hair, promotes relationships etc...

The point is, where does Mary Sue stop and the OC start? Every OFC I introduce must have something special about them, because otherwise there's no point including them in the story. An OFC must be important to the plot in some way. Of course, a blatant Mary Sue is easily spotted, but how much influence is an OFC allowed to have on the canon characters before she turns into a Mary Sue?

Questions, questions...