Being a reader and writer of fanfiction myself, I’m undecided when it comes to the usefulness of the Mary Sue concept, especially when it’s applied to original fiction. First of all, it’s too inexact to be of much use, since it can mean, a) a self-insert character in original fiction, b) a self-insert character in fanfiction, c) any original character in fanfiction, d) any character the reviewer dislikes, or e) a character that is the author’s pet and spoilt and entitled brainchild, and the black hole around which the narrative revolves, not by virtue of their own abilities as much as by the author’s word. I tend to go for the last definition, though it makes it a lot more difficult to issue general statements about what a Mary Sue character is or is not. But you recognise them when you see them.
Any online Mary-Sue tests I’ve seen seem to address generalised symptoms rather than the original, individual character, but removing the symptoms doesn’t address the problem. So you’ve changed Arwen’s baby sister’s eyes from colour-changing to a dull dishwater grey, removed her sparkly unicorn pet and made her plain-looking for an Elf, but she still insinuates herself as the tenth member of the Fellowship, emoes about the star-crossed love that she and Legolas Bloomleaf bear each other, steals all the scenes and saves the free world just by existing. She still is a Mary Sue.
Conversely, some characters who bear all the signs of being self-inserts still seem to be bearable. Harriet Vane, for instance. And there’s the dilemma, is she a bearable character because she isn’t a Mary Sue, or is she a Mary Sue and DL Sayers an incredibly talented author?
It probably follows from my definition that I hesitate to use the term Mary Sue, though I did it and meant it after a May weekend of skimming through three PDF’s of the Twilight saga. Though the author wanted to give the impression that Bella was just a plain, shy, bookish and clumsy teenage girl, she manages to have her entire school awestruck by her presence, an entire vampire clan and a werewolf tribe at her beck and call (apart from some token jealous bitches) – for no very good reason at all except that the author wants it to be. And it went on like that for four books, with Stephenie Meyer pulling strings, lowering mountains and acting as Bella’s curling parent. This is highlit by Bella’s complete passiveness; she stands there and the entire story happens around her, without her having to lift a finger.
“We are the Vampire Police and now we will kill you for exposing our secrets to the world! Except no, we won’t, good-bye and safe journey home!”
“Holy crow, Mom will totally kill me for getting married straight out of highschool – except that she doesn’t because she adores my boyfriend and thinks I’m so mature.”
“Woe is me, if I become a vampire I’ll never be able to meet my family again – except now Dad is sitting in the living room in Casa Cullen, watches football with Emmett and doesn’t suspect a thing.”
Count me as one who got extremely disappointed that Bella’s clumsiness (classic Mary Sue flaw, that; a flaw that doesn’t affect the story and only makes the character look cute) wasn’t the sign of a slowly advancing and inoperable brain tumour. She could have died a noble Mary Sue death and united grieving humans, werewolves and vampires at her grave.
Perhaps Batgirl/Oracle would be a better example?