Because some people out there really can't take the hint.
1. Remember to have long, shiny, silky hair, that you either let down or wear in braids.
2. Preferably, it is neon green or something like that. If you can't go for that, then at least golden-blond or raven-black or as red as a ripe carrot.
3. Of course, it should come from magic or you should just be born that way. Dye is unacceptable.
4. Your eyes must be green or purple. See number three.
5. You must have an extremely unusual name and a least four middle names.
6. Any teachers who show favoritism toward your enemy are the Evil Overlord.
7. Of course, any teachers that show favoritism toward you are the Wise Old Mentor.
8. Describe all your clothes in lavish, excessive detail.
9. Develop unusual powers at an alarming rate—one a month or so is best, more quickly if you can manage it.
10. You are always right. Anyone who says otherwise is clearly villainous or severely brain-damaged. React accordingly to any criticism you might receive or perceive.
11. Shoehorn in on everybody else's glory. If Secondary Character #3 makes a spectacular speech that results in a standing ovation, you must immediately make a speech that gets you an even better reaction.
12. Warp the story so that everybody's lives revolves around you. If the other characters do not spend every waking moment wondering about your well-being or pandering to your every whim, you are doing something wrong.
13. As an addendum to #4, your eyes can be any color (see: Bella Swan), but they must either change color or flash noticeably when you become emotional, or simply be remarked upon as often as possible without a plot-relevant reason for this to be so (i.e., Harry Potter does not meet this criteria because his eyes are remarked upon because they remind people of his mother, not because they are OMG so pretty).
14. You should seriously consider getting some form of cute animal familiar. Bonus points will be given if you are the only character in the story to do this.
15. Always be full of self-righteous pride and moral superiority. Your actions are always good and right, regardless of how similar they might be to the Evil Side. (I AM LOOKING AT YOU, ROSE POTTER.)
16. You must have more than one love interest at the same time, which you spend about a book wondering what to do about. If you can not get more than one, he must be immortal, supernatural, etc. and you must have difficulties because of that. Bonus points if you have two love interests one or both of which is immortal and supernatural (coughcoughcough)
17. You must faint much more often than anyone else in the book, and still be considered a tough heroine that can do everything herself.
18. You must be described as 'loner' or 'never smiling' and still get amazing surges of emotion every other chapter.
19. If you are very short tempered, prove that by only getting angry at the Evil Overlord who killed your parents and gave you a horrible life.
20. You must be flawless.
21. You must not have fear, except for the huge things, like the Evil Overlord hunting you down For that, you are permitted to quake in your boots.
22. You must have a small scar that is all that is left over from your skirmish with the Evil Overlord. However, it must not make you ugly. It must enhance your cuteness.
23. You must win at EVERYTHING. Everything must go perfect; all the odds must be ever in your favor. If you don't win or succeed, the game/event/quest is obviously rigged and you must go find the person who "tricked" you.
24. You are not allowed to speak the word "failure." It's pretty useless if you're not allowed to fail. (See rule #23.) And for that matter, the words "lose" and "tragedy" are banned too.
25. Always go around spouting kind, sweet words of wisdom whenever people need it. Use this especially on your enemies, because they will be so shocked that somebody is saying nice things to them, coming from a long and storied past, that they will be sure to give in!
26. You must have the ability to make people soften/mellow during the duration of your story.
27. Following rule #25 you must have it in your power to forgive/see your enemies in a good light and (this is optional) recruit them to the good side.
28. You must have endless upgrades/powerups.
29. You must actually be a princess in disguise.
30. You must be stunningly gorgeous yet completely skilled in the ways of the bow and arrow.
31. You must only fall in love with 2 or 3 guys who are, of course, the most poular/handsome/sweet guys in the world, but every other guy has to fall in love with you. If they don't, they are either a complete imbecile or a warped, twisted, unwhole villian MWA HA HA who is in cahoots with the Sugarbowl.
32. If this is a fantasy story, you must be an elf or some such amzing magical creature. If not, then you must have a frighteningly large herd of admirers and someone who is completely jealous of you and there will be a big showdown but you will use your charm and wisdom, and she will become another one of your retinue of followers.
33. Similarly, there may also be a girl with an equally large herd of admirers who is a complete cheese puff but after a large feud/battle/showdown she will become your best friend.
34. You must be uber athletic, amazingly smart, witty, poular, and skilled at everything. If you so much as get below an A+ on your report card the warped, twisted, unwhole villian MWA HA HA who is in cahoots with the Sugarbowl will swoop down and strip from you your Mary Sue title.
35. You must either be really tall and shockingly gorgeous, or really short and incredibly cute.
36. You must spell bettter than I do.
37. You cannot be on "the bad side" or in cahoots with the warped, twisted, unwhole, villian MWA HA HA who is in cahoots with the Sugarbowl. Shame, that.
38. You must wear some form of make-up, yet still look gorgeous without it.
39. You must do one little tiny bad thing, then get so guilty you did it, you must tell on yourself and be viewed as an angel for telling.
40. You must save at least one thing, whether it's a cat or the whole world.
41. You must be the only person in your giant herd of followers to be able to communicate with magical beings, use magical things, etc.
42. Anyone else in your herd who can do these things must be killed off.
43. Early on.
45. Except you can't say that.
46. So you put on a brave face.
47. Although he was going to be your love interest
48. But in that case, you're allowed to cry
49. But you still don't, because you have Unicorn-Man (coughFablehavencough)
50. You only eat cheeseburgers
51. Or candy bars
52. But you're still really thin
53. Because your metabolism was sped up
54. By a magical spell
55. You can't die
56. Because that would be problematic
57. Actually this goes for any book
58. Except maybe one written about the afterlife
59. Or in third person
60. Your friends all worship you
61. You're an orphan
62. Or your parents are the warped, twisted, unwhole villains MWA HA HA who are in cahoots with the Sugarbowl and want you to join the Dark Side.
63. You must be a vegetarian and rub it in everybody's face that you don't eat meat, that all people who eat meat are barbarous, horrible, evil, twisted, MWA HA HA etc. and your sheep people agree, become vegetarians, and then you up them by becoming vegan. (optional)
64. If you are in a fantasy book and are NOT a magical creature, then you must be the one human in a whole slew of creatures and have everyone think you're amazing for being human.
65. You must be the Chosen One. Variations on this theme are acceptable.
66. You must only get into trouble accidentally, through no fault of your own. It should always be random or somebody else's fault.
67. When you do get into trouble, you should usually be rescued or heavily aided by another person. Everyone must think that you are amazing for getting out/solving the problem afterwards when really, you didn't do much at all.
68. Deny that you are amazing. Be bashful about it. This will only make people compliment you more.
69. Suffer some tragic loss at some point in the story (preferably a family member, love interest, or aforesaid wise old mentor) and grieve for days about it. If the tragically lost is a love interest, proceed to almost immediately find a new supernatural/immortal/handsome boyfriend.
70. Your boyf is the hottest one there is.
71. You make straight girls and gay guys fall in love with you. Alternatives to this include having the ability to turn on literally anybody with a look.
72. Your familiar/pet must be some sort of fantastical creature
73. With an adorable name
74. And it looks like that one Pokemon you can't quite remember the name of at the moment
75. But you still get away with it.
76. If inhabiting a fanfiction: You manage to secure the most sought-after DLI for your boyf. And happen to be related to the most badass of characters.
77. You surpass everyone in your badassery.
78. You can transform into different animals.
79. But only the really cool ones
80. Like a tiger
81. With wings
82. And miscellaneuos other awesomenesses.
83. You have super-destructive powers
84. That could probably blow up the world
85. And you've totally had them from birth
86. But you'd never be a villain
87. Because that's just plain horrible.
88. You must be smart. As in, spectacularly smart. On the same level as the genius of the team. But you never really show it, because you're dithering over your supermegafoxyawesomehot love interests,
89. Boy's names always sound cute on girls!
90. Never mind the Laws of the Universe, you will always win.
91. Not to sound racist or anything, but you are white. And if you are not white, you will constantly harp on that fact and become some sort of crusader for civil rights.
92. You are always stronger than the boys.
93. You are the only female in a huge cast of exceptionally attractive male characters who all happen to be hopelessly in love with you.
94. And you can whoop all of their asses blindfolded.
95. Even if you never do anything, you are always regarded as a hero.
96. You have deep vats full of compassion and mercy that you dump on villains to turn them over to the Side of Good.
97. You are a cross-breed/hybrid of some kind. Bonus points if you manage to become half of every species you encounter, so much so that you shouldn't really even be recognized as a human anymore.
98. But you never show that many outward traits, aside from a tail. Or ears. Or wings. All three if you can get them.
99. You managed to secure all the strengths of all of these creatures, but none of the weaknesses.
And the final step to becoming a Mary Sue...
100. Never admit to it. Pin the blame on other characters, even the author. But you are the blameless innocent absolutely guileless hunk of text that is the victim here, obviously.
So, that's my rage for the day.
Notes: These traits by themselves do not a Mary Sue make. Use them in moderation along with some original ones and you've got yourself a respectable character.