As some people have mentioned that they would like to see the fabled checklist...
The Mary Sue Checklist
In order of common Plot Chronology Occurrence
1) The Mary Sue character exists for the Suethor's wish fulfillment. The Sue looks like the author wishes she looked, acts like the way she wishes she could, says what the author wishes she could say, and creates an environment within the canon in which things that the Suethor wishes would have happened in the canon, does. The Mary Sue is sometimes the gender or sexual orientation which the author wishes (s)he was, or feels is fashionable.
The Mary Sue is the Id, as according to Freud (1920; 1923), let loose to run rampant, to hedonistically enjoy a fictional world with no fear of repercussion; she is an avatar who experiences the sexual interaction with fictional beings that the author can never access; she is the putty that fills the gaps that separate the writer's own reality with that of their favourite fiction (Jenkins, 2007, 173).
2) There is a sense or tone of whimsy and /or self depreciation in the text.
Most Mary Sue characters are written to be “competent. But not too competent; she’s still a little girl inside, and there’s a sense that at least some of what she accomplishes is done by accident or luck” (Pflieger 11). Suethors experimenting with wish fulfilment don’t often also experiment in agency and therefore introduce chance to further the plot, which lends an air of the narrative pressing forward despite the Mary Sue character, rather than because of it.
3) The Mary Sue’s appearance, personality, and even scent are often divulged in detail within the exposition of the story; Mary Sues often have physical traits that are a biological impossibility, such as eyes that are violet or shift colour, pointed ears, the limbs or ears of an animal, or hair colour that is not within the normal spectrum of shades.
As the Mary Sue is often the first example of attempting to create an original character for many authors, they feel it necessary to load exposition and description into the text at the earliest opportunity. As professional writers rarely describe their own characters so blatantly, this is considered a mark of an amateur and a turn off for most readers. However, due to the nature of media text making, the characters in a particular text are usually also extremely engaging, intelligent, interesting, brave, beautiful and talented. A Mary Sue, therefore, must be more perfect than the characters in order to win their affection, admiration, and interest (Pflieger 10), and thus must be described so thoroughly in order to compare to the canonical characters and / or overshadow them.
3) The Mary Sue often was given a name with a hidden or relevant meaning, or that simply “sounds cool”, with no regard for ethnic tradition or likelihood.
Often this is an attempt to mask that the Suethor is writing about an idealized form of herself by fitting the character with an idealized form of her own name, or a name with a significant meaning which they feel describes themselves, with no regard for the ethnic tradition / genesis of the name itself.
4) The Mary Sue is a character either previously unrecognized by the fandom's central protagonists and antagonists before the commencement of the story, or drops in from 'reality' into the fandom.
The origin of the Mary Sue character depends on the role she is to play in the story. If she is to act as meta-commenter, to be wish fulfillment for the Suethor’s desire to see specific events or pairings occurring in the source text, then often she arrives from our own ‘real’ reality to influence the events and characters of the canon text. If she is meant to engage in a relationship with the characters and become an integral part of the ensemble cast, then she is often introduced as having existed in the source text all along, the far flung relation or long-lost-something to one of the canonical characters, a previously unseen or ignored co-worker, or a special consultant / agent / student / etc transferred in or visiting.
5) The Mary Sues’ abilities / magical powers / skills / talents not only outdo the canonical characters (even if, or especially if, the canonical characters are the most powerful / talented / skilled at this particular ability) but often eclipse them. The Mary Sue directly copies the skill and is better at it. The Mary Sue is adept at any sort of fighting or magic wielded by the canonical characters, often surpassing the canonical characters in ability with little to no practice or previous experience.
To be a functioning and useful part of the canonical world, the Mary Sue must have the abilities to earn the respect and endorsement of the canonical characters. Without any sort of power – and any sort of power useful to the characters, therefore similar or exceptional – the Mary Sue is relegated to the position of innocent and useless bystander, and therefore cannot engage with her desired character or fulfill her particular wish.
6) The Mary Sue is immediately accepted into the canonical character's inner circle of friends and confidants, no matter how unsociable or closely guarded said circle of friends is in the canon.
6.5) If the Mary Sue is related to a major character that is not the romantic interest, then said relation is often used as a haphazard excuse to introduce the Sue to the canonical characters/ future romantic interest.
If the Mary Sue does not gain entrance, there can be no relationship, and therefore no plot, and thus no story. Mary Sue stories focus on relationships more than plot; therefore, while a long and complicated narrative in which the Mary Sue gains the friendship, trust, and eventual acceptance of the canon characters could be written, the Suethor bypasses this approach and imposes the friendship in order to move swiftly to the relationship portion of the narrative.
7) The Mary Sue acquires friends or romantic interests / partners among the canon characters, blocking or disrupting the canonical friendships, partnerships and romantic relationships within the canon.
Mary Sues exist so the Suethor can figure centrally in the most important relationships – platonic or romantic – and therefore characters who are canonically linked must be severed or blocked from each other by Mary Sue’s mere presence in order to facilitate the wish fulfilment that is the Suethor’s primary motivation.
8) The surrounding canon characters are made to act out of character by the Mary Sue’s presence.
Pflieger points out that this is called, by most fan craft readers, “character rape” (Pflieger 6), and occurs because character’s personalities have developed in relation to only those characters that exist within the show. In adding an unknown quantity, the chemistry of the ensemble must change. Some mistakes in out-of-character instances, however, could also be traced back to the Suethor’s neophyte writer status, in that she has yet to master the subtly of writing that allows a character to remain in character, even when written by an outside party like a fan crafter.
9) The Mary Sue robs the strong female characters of their rational agency, turning them into catty bitches.
Again, this could be traced to the Suethor’s newness to crafting, but also to the feminine inclination for competitiveness. Strong, capable female characters threaten the Mary Sue’s agency, importance, and place in her beloved’s heart and therefore they must be removed or made undesirable. Teenage girls, the primary bulk of Suethors, are also living in a phase of life where all women are potential competition and bullying is a fact of life; all girls feel persecuted, and the existence of a character that could challenge a Mary Sue’s ability to form relationships with the desired characters / romantic target, is therefore a potential threat and must be neutralized via changing their characterization.
10) The Mary Sue is sexually attractive to all canon characters, regardless of their
sexual orientation or availability.
The age at which most Suethors are beginning to write Mary Sues is also the age wherein personal confidence and self exploration begins, as well as the blossoming realization of sexuality and the power women can hold over sexual partners in regards to sex and attraction. The Suethor uses the Mary Sue story to experiment and understand the power they have as young women over other sexual beings, and to comprehend their own sexual urges and fetishes by playing them out in a fictionalized –and therefore safe – space.
11) The Mary Sue is often ardently desired by the villain despite there being no advantage or reason for the villain to want said Mary Sue.
Again, the Suethor is experimenting with her own sexual or intellectual attractiveness, and in most media texts, the villain seeks to defeat the hero by taking away or destroying everything that the hero relies upon for stability and happiness. That includes the Mary Sue, in a Mary Sue story, so the villain must desire her, too.
12) The Mary Sue is often the reason for the main conflict in the plot, whether inadvertently or as a central figure. Any special destiny, quest, or plot arc possessed by the central character of the canon is often usurped by the Mary Sue.
The Mary Sue has often been called the ‘black hole of plot’ or the ‘thief of mythos’
(see chapter 4); for example, if Harry Potter is the child spoken of in the prophecy, then the Mary Sue is the real child the prophecy means. As the plot of a media text surrounds a canonical character, the Mary Sue must usurp that plot in order to be included in the sphere of her desired character’s relations. Otherwise, Mary Sue has no reason for interacting with them.
13) The Mary Sue is witty and snarkish, and no matter how lame said witticisms are, the canonical characters akin the Sue's sense of humour to that which is quite practiced and enjoyable.
This often goes back to the Mary Sue’s neophyte writer status. Media texts often possess their own particular forms of dialogue and humour structure, which the Suethor (or all fan crafters for that matter) isn’t always able to replicate successfully.
14) The Mary Sue is often able to shift shapes, or has a friend or magical pet that can shift shapes.
As noted above, most young Suethors are at a stage in their lives where their bodies, minds, understanding, emotional relationships, and life patterns are shifting due to puberty, sexual awakening, educational expansion, etc. Relying on a constant, imaginary friend may be a holdover from younger childhood fantasies, or it may also be an unconscious signifier of the turmoil that such changes cause within the Suethors. Such creatures often accompany canonical heroes, especially in the fantasy literature aimed at the Suethor demographic – Harry Potter has Hedwig, SailorMoon has Luna – and therefore the Mary Sue must have one also, and one that, like the Mary Sue herself, is more.
15) As the Mary Sue is the author’s avatar, the Mary Sue enters the world of canon with the same understanding of character, plot, and universe as the Suethor; the Mary Sue therefore has incredible insight into the lives and workings, the desires and dreams and fears, of the characters with which she is interacting and often serves as a conduit for advice that the Suethor wishes to impart on the characters. The Mary Sue ‘understands everything’. The advice considered, the canon characters often behaviour is often altered to accommodate the Suethor’s desired changes.
16) The Mary Sue has a tragic past that proceeds the story, or something traumatic and mentally scarring occurs mid-story that they 'get over' cheerily, suffering neither post-traumatic stress, Stockholm syndrome, or other disorders or phobias brought on by their past experiences. Any scars or marks that result from the trauma are 'cool', or are interesting places or shapes, and are often aesthetically tasteful. Sues are rarely ashamed by said scars or marks, or associate them only lightly with the trauma.
16.5) Mary Sues mope, brood, or pout, but only for as long as it takes for the canonical characters to distract her. Long-term guilt or pain is rare in a Sue.
While the hurt/comfort genre of fan crafting is popular, it is rare to see in a Mary Sue story. Most Suethors are interested in the acts – especially the sexually violent acts – as methods of exploring personal fetishes, blooming sexuality, and imagined experiences, without concern for the fall out. As the Mary Sue is the Id, it is unconcerned with suffering and emotional strife, and as these things are undesirable in real life, they are made to cease to exist in the Mary Sue realm.
17) The Mary Sue is the centre of the plot, the key player in any battle, and is the only one who can 'save the day' by virtue of her wonderfulness.
17.5) And in doing so, the Mary Sue ‘ties up loose ends’, filling in plot holes from canon, getting characters who the Suethor believes are romantically destined for one another together, or forces an ending to the major plot arc from canon in a satisfying (to the Suethor) and happy way.
Paula Smith notes that “the truest mark of a Mary Sue is not how she’s described or what she does, but the effect the sheer fact of her existence in the story has on the other characters in the story” (Smith 1996). Pat Pflieger adds, “In these stories, Mary Sue is the centre of the known universe” (Pflieger 6). For example, in my 2003 fan fiction Harry Potter and the New Neighbour, my Mary Sue Anathemia Oldwyn uses her vampiric powers to force Peter Pettigrew to confess to the murder of the Potters, thus clearing Sirius Black’s name and freeing him from Azkaban Prison.
18) Somehow the Mary Sue character 'saves the day' in unlikely ways that leave the canon characters stunned and amazed, often at the cost of her own life / health / magic powers / happiness / love / ability to stay with the characters or in the fandom.
If Mary Sue must be more than the canonical characters in order to maintain her relationship with them, then the method in which she saves the day must also be more.
19) Yet somehow there is a happily ever after wherein the Sue and the Love Interest are reunited, often affirming their relationship through marriage and procreation, therefore supporting the patriarchal hegemony that a powerful female character like a Mary Sue could otherwise challenge.
19.5) Unless the Mary Sue dies, in which case her death is tragic, meaningful, beautiful, and offers the characters that the Suethor is most directly interested from the canon a lesson / morale / momento to take with them.
Pflieger explains that there are many practical reasons for the Mary Sue to perish: like the many deaths of the women-of-the-week in canonical series, “it clears the decks for next week”; “there is the angst value”; the way that nobility in death (or of death) has fascinated writers for centuries (Pflieger 8). Most importantly, she says, “death keeps Mary Sue memorable to those who really interest the author [the characters], fixing her in their minds as in amber, at the peak of her loveliness and strength and nobility and virtue” (Pflieger 8-9). Being dead also means that Mary Sue cannot ever grow un-lovely, cannot ever make mistakes, bungle, prove herself incompetent, and otherwise ruin the perfection of her actions (Pliefer 9). It also spares the love interest characters from having to actually have a relationship with her, sparing them a messy breakup or divorce, or falling out of love (Pflieger 9). It also undoes the trauma of the events a Mary Sue may have had to suffer through, and also allows the Suethor to feel content that while she has exposed her fantasies to the world, she has effectively closed them off from her own psyche by eliminating the chief avatar of said desires. The wish has been experienced, explored, satisfied, seduction has been practiced without the need for the natural consequences of it (sex, pregnancy, and/or commitment, whichever is scariest to the Suethor) and fulfilled and now can be neatly filed away with the deletion of the evidence.
 And drawn from such a myriad of fan crafts, essays, online forum discussions and blog entries over the last eighteen years that I could not even begin to hope to properly cite all of them. Let us assume, then, unless otherwise directly noted, that this information came from somewhere and somewhen and have accumulated like strata, piling and weighing down the coal until a diamond kernel of understanding has emerged.
 Wherein the majority of the plot occurs around an injury or illness that is inflicted on one character. The other characters must care for the injured character, thus provoking an emotional revelation, a catharsis for the readers, and usually the achievement of character expressing their feelings for one another – previously hidden or un-acted upon – and ending the story are paired off, usually romantically but sometimes in a platonic, but deep, friendship.