For myself, there are three characters that I absolutely love, that technically are "Sues": Sherlock Holmes, Dr. Who and Honor Harrington (grumblegrumblenextbookinfreakingJulygrumble) All three are badly overpowered in their chosen expertiese, all three have no true equal, and all three are highly idealized. Honor, in addition to being the BEST ADMIRAL EVAR!!one1! has a list of titles longer than most libraries, and Nimitz, her telepathic/empathic treecat that also gifts her with psychic powers in a universe where humans are not psychic (and that kind of universe-breaking is a big hallmark of Sueism) And then there is the Doctor. Ignoring the Tardis, ignoring the large stable of women desperate to play tonsel hockey with him, even ignoring the fact that he is effectively immortal, he carries a deus ex machina around in his pocket. Sonic Screwdriver, anyone?
And yet...they work. And they are hugely loved by the people who read them. People almost rioted when Sherlock died. I only found out that Honor was to die after Weber decided not to kill her (and I will never trust him again. Because you do that once, you can do that again, and it was bad enough loosing LaFollet last book. *sniff*) and the Doctor has survived longer than most socialist governments. What the hell?
Which brings me to the first criteria for a true Sue: She is an extraordinary character without an equally extraordinary plot.
First criteria for the exceptional plot are circumstances no other character could meet, that will still carry a high cost for the exceptional character who meets them.
The first time we meet Honor Harrington, she's an impressionable, beautiful, impossibly young ship captain on her first command, with her
And this cannot be done. She tries, which proves to us that this cannot be done by normal means, given the amount of foot-dragging, name calling and sheer technological effort she has to go through to get it half right.
And then she realizes that the space station is about to be invaded by the People's Republic of Haven, who will have more than one ship. And these ships will not have had their armament ripped out by an idiot and replaced by untested theoretical junk. And she has to figure out how to stop this when all of her resources are tied up in policing the star system (and quelling an incidental alien riot). And throughout this story, so much has gone wrong that the only reason we know she survives this is there is a second book.
It would take an exceptional human being to succeed under these circumstances. The plot of On Basilisk Station demands an extraordinary character with a specific skill-set that Honor happens to have. She has neither more skills, nor fewer skills, than what is needed to meet her challenge. Except for the tree-cat. That's the second book.
The height of Honor's success is pretty astronomical, but she goes pretty low too. She earns everything she gets, and you kind of get the sense that the accolades are just kind of like wallpapering over a wound. The titles don't trade over for the amputated arm, the lost eye, the nerves in her face, or the large number of friends she's had to bury. The exceptional plot makes the exceptional hero pay for every success.
Next criteria for the exceptional plot: The circumstances would occur with or without the main character's existance.
In Basilisk, the Republic of Haven knows this system is badly policed and is going to invade it. They engeneered half the problems Honor is dealing with, expecting her to so overextended (which she very nearly is) that she can't handle them. However, they planned to have a corrupt, ineffective officer on station, and not hyper-efficient Honor. So instead of overwhelmed, overextended and in need of help, they invaded a system with an almost-overwhelmed, almost-overextended, but I don't need your help at all, thank you commander, who was very happy to kick their ass ... at great expense to her ship, her crew and herself. Honor was able to do the impossible--police the system for her monarch and hold it against an invasion--but at such a great cost to her person that you truly believe if ANYONE ELSE IN THE GALEXY were in charge of Basilisk, the day would have been lost. This is a pattern that holds in every book with her in it. Honor wins...at great cost to herself. In the later books, Honor is often the only victor on her side of the line--her kingdom's other forces lose. Badly.
Honor is not the cause of the problems. She's the solution.
Which brings us to point number three in "Extraordinary Plot": the rest of the cast. Antagonist, support cast and love interest are all of huge importance.
A strong antagonist, stronger than the main character, is necessary. They need to be capable of taking the fight to the main character's doorstep. In other words, they need to demand the extraordinary character exist.
However, it's the support cast that saves the day. These are the real people. Gentle, flawed, rough around the edges, very obviously incapable of reaching the main character's level of success (otherwise we wouldn't need her) and yet equally obviously necessary for the main character to succeed. They are to remind her that she, too, is human. They disagree with her. Sometimes they spoke her plans. Sometimes they are extraordinary in their own right. My favorite pairing in the Honorverse is Scotty and Harkness, the clean cut young midshipman assigned to search ships for contraband, and the grizzled non-com who somehow is an expert at hiding contraband. There are lives and relationships around Honor that transcend her. Not as soaringly awesome as she, but just as fun and entertaining, and not always awed by her. As the books evolve, you realize she needs her staff as vitally as they need her, and when they start to die in battles, you believe that each death wounds her.
The cast of the exceptional plot are not there for the exceptional character. Rather, she is there because of them. She's not wholly their protector, but they're why she has involved herself. However the novel starts out, and whatever motivates the initial steps towards conflict, by the end she fights for her friends and her family, and to stop the antagonist from harming anyone else. And often, her friends and family are fighting for the same reason.
And now, because I knew I'd have to do it eventually and I want to get it over with...Bella Swan, ladies and gentlemen, and why I felt she was a good example:
Point one: Exceptional Plot requires circumstances no one else can meet.
Bella Swan is not that exceptional. The novels go out of their way to prove it. What makes her exceptional is Edward's inability to read her mind (dense skull?) and his falling in love with her. And yet it is heavily implied that there is some amopheous something that makes Bella exceptional. We never find out what this is. Without this ever being identified, we believe that any girl with a tin hat and perfume could have gotten Edward to fall in love with her. All Bella did to earn his love and her exceptional status was go to Forks, be the new girl and smell pretty.
The pivotal moment in her story is when Edward rescues her from being smushed by a van. So to the criteria of new girl, smell pretty, and tin hat we add "useless". What motivates Edward to fall helplessly in love with her is Bella being helpless. This is where she gets propelled into Sue-dom without prayer of recovery. When the negative character traits are what gets rewarded, the rest of us stare in a mixture of envy and betrayed disbelief. The world, we know, does not work this way. We wish it would, but we know it won't. Bella is an unambitious girl with no hopes, no dreams, no talents and no interests beyond snogging her boyfriend. There is nothing exceptional about her, save that she is in this story.
Point two: The exceptional circumstances would occur with or without the main character's existance.
Vampire. Baseball. I could stop right here.I won't, but I could.
The biggest problem with Bella Swan is that she causes all her problems. Her physical danger is the result of her choice in boyfriend. Everything at risk is because she made (bad) choices and is reaping the concequences. At no point does any outside force come along and place her or her loved ones at risk for any reason other than her actions and choices. In book one, James attacks because he wants her, and he almost kills her because she chooses to be alone with him. Book two, Victoria wants to kill her because Edward killed James (because James wanted to kill her) and Edward wants to kill himself because he thinks Bella killed herself. Book three, Victoria wants to kill Bella because of James, and Jacob gets involved because Bella was a manipulative bitch. Book Four, the baby happens due to sex, and the Volturi try to kill the Cullens because of the baby (and they also want Bella).
If you removed Bella from the story, would the Cullens have ever been in danger? No. The story revolves around Bella. She's the cause of their problems AND (sometimes) the solution. Bella is a Sue, when she is ordinary, and Honor is not, though she is extraordinary, because the villain's motiation is due to Bella's actions, whereas in the Honorverse, the bad guys choose to act without any imput from Honor, and she just gets in their way.
Criteria three: The extraordinary character has a cast strong enough to support her.
Quick! What's one of Rosalie's hobbies? How about Bella's Dad? What does he do in his spare time? His job? Does he arrest anybody? Put drunks in the drunk tank? How about Bella's Mom? Does Alice have any hobbies that don't directly involve Bella? What does Esme do other than "be motherly?" How about the Volturi? What do they do for the vampires that make them the leaders?
None of these things are ever mentioned. None of these characters have a life past Bella Swan. Nor does Bella have a life beyond Edward. As I said earlier, she has no ambitions, plans, talents or joys other than her boyfriend. Her supporting cast either ALWAYS rescue her or are totally rescued by her (Breaking Dawn).
One of the most interesting moments in the Honorverse was when Harkness is told that if he will stop hacking the BuPers computers, they will assign him to whatever berth Scotty gets for the rest of his career. Because he was doing that the entire time. This is like five novels into the series, I believe. At one point he does rescue Honor using his ill-gotten skills as a hacker. All the characters in the Honorverse have lives and interests that have nothing to do with Honor. You see these glimpses of other stories and become facinated.
This never happens with Bella. Esme doesn't make stained glass lampshades or knit sweaters or council small children in her spare time. Carslile is the only Cullen with a job, and that makes him one of the stronger characters. Rosalie has no life other than hating Bella and wanting a baby. No one does anything unexpected. You don't catch Emmett watching Jane Austen movies when he thinks no one is watching him. The villians have no goals that don't directly effect Bella, either. Aro is a collecter because he wants Bella. Caius is...what the hell are either Caius or Marcus, other than as door-stops for Aro? No one has a life outside of Bella Swan. There are no details that cannot be immediately explained by the text. The cast, therefor, is one-dimensional and weak.
So to sum up? A Mary Sue is an extraordinary character, without an extraordinary plot. And an extra-ordinary plot is defined as:
1. A set of circumstances that demand an extraordinary character to fill them.
2. A set of circumstances that would occur with or without the main character's existance
3. A cast of people who have lives and goals beyond the main character, who are allowed to succeed fail, and/or be harmed with or without the MC's imput--who thus make her success all the more important.