After reading everything and thinking about it a bit I've come to this conclusion:
- A power fantasy/wish fulfillment character that has no arc over the course of the story.
Typically caused by bad writing (not being able to setup or understand the need for a character arc) but certainly not limited to self-inserts.
Maybe we should start judging Mary Sue's on a sliding scale of how much they break the rules of their own universe (from slightly OP to Deus Ex Machina) related to how boring they are.
- A power fantasy/wish fulfillment character that has no arc over the course of the story.
Typically caused by bad writing (not being able to setup or understand the need for a character arc) but certainly not limited to self-inserts.
Maybe we should start judging Mary Sue's on a sliding scale of how much they break the rules of their own universe (from slightly OP to Deus Ex Machina) related to how boring they are.