It's hard to say when the Mary Sue witch hunt started in this modern time. I'm sure that it's happened before when the term was first established, but it seems as though lately it has gotten way out of hand. You can't swing a dead cat in a literature world without hitting someone that is accusing one character or another as a Mary Sue and is therefore crap.
In the link above you'll find the basics of what constitute a Mary Sue, but like some other terms it seems to have been losing its original meaning and can mean anything from a character that seems utterly perfect, one that is too similar to the author, or one that is extremely bland with a few similarities to the author. It is being abused pretty badly these days, and it is often being applied incorrectly.
For the purposes of this article, I'll be identifying the idealized overly perfect version of the Mary Sue plague. The character that most often gets this missile launched at them is a competent, skilled, likable and moral person who is most often the protagonist. If a male then he will often have women fawning over him left and right.
Now, if you haven't shut yourself up in a Vault with no form of media whatsoever, then I'm sure you've heard of this type of character at least once. If you haven't then I can only assume you just broke out of a concrete bunker where all human contact came in the form of someone slipping your tray of food in through a thin metal slot beneath the door, probably using you as a form of experiment. (If this is the case, then let me know how you learned to read and enjoy your freedom.)
Anyway, you may wonder why this type of character mold is so common, and the answer is simple: Most people like reading about strong characters that overcome resistance put before them. The adventure is a test of their mettle, and if we like them then we wish for them to succeed. This is very basic of course, but it still bears mentioning.
This is one of the absolute key differences between a strong, competent character and an idealized Mary Sue: If it's a Sue, then everything is easy, and therefore boring. There is no challenge, and thus no tension or room for growth.
Many people criticize characters as Mary Sues on the grounds that they are too perfect, when in reality this might not be the case. Often I've seen people advise others "Your character has to have flaws." This could be in terms of competence, skills or morality. I don't find this to be the case. At this point you're not working on an organic character with a backstory and personality, you're filling out a check sheet. A formula can help establish a framework of a character, but readers can pick up on if you're doing this and it feels fake. Some might like it, but I don't believe you should go down a check list to make someone.
This can also lead to the Anti-Sue, which is an obvious effort to be very flawed, not terribly skilled and morally questionable. In most circles this translates to something known as "An unlikable jerk." They aren't terribly sympathetic. This can be done, but there still has to be something to like about the character, or at the very least identify with. I don't know about most of you, but I can't stand characters that whine and cower as everything is going on around them, complaining that life sucks. I don't identify with people like that. I prefer characters that suck it up and forge on against opposition in spite of their problems.
I call this "A butt-kicking hero." This is typically the most popular type of protagonist, by the very virtue that they are people we aspire to be like. They set a high bar that we hope to one day reach. Many of my favorite characters in books and movies are of course chiseled warriors that do what needs to be done in order to finish the job.
Now, this is NOT to imply that all characters should be the paragons of goodness and valor. There are many, many levels of character and all can be sympathetic and likable, or at the very least interesting if they are implemented well. This is key in any story. But like I said, we're working around Mary Sues at the moment.
Often characters like Tarzan, Conan and other hunky warriors are accused of being Mary Sues, and for a time I myself was confused, because this is where the lines really blur. In pulp you see guys who are the best of the best, matched only by perhaps a few other men in the world. They are called on to fight vicious armies, shadowy warriors, clanking robots, slithering beasts, anything you can think of! So I wondered heavily as to why I like these guys and not some others. And that's when the revelation above hit me: These guys succeed against their enemies, but it's hard!
Everything is stacked against the heroes. They are outnumbered, given inferior equipment and weapons, infiltrating enemy territory where they have the home turf advantage, and manage to win only by the skin of their teeth after exhausting amounts of effort. Their awesomeness is measured only by the adversaries and obstacles they must overcome.
A Sue on the other hand doesn't have to deal with all of that pesky effort stuff. They master things quickly and instantly without requiring much practice or learning. Now, some characters can learn things darned quickly, and that's okay, but the big problem here is if the conflict is just a walk in the park. I recall awhile back reading a Western that I initially thought was a Louis L'amour, but quickly picked up on the fact that it was a different writer due to the writing style and language. Of course, I could have just looked at the cover to see who had actually written the sodding thing, but this somehow escaped me. Don't ask how, even I don't know.
Anyway, the protagonist was a beefy strong gunslinger with hot Native American wife. At first this seems like a decent hero, but what bothered me was how they had friends absolutely everywhere in positions of power who went out of their way to help the hero and step on the bad guys, or even guys just trying to do their job who happened to not be part of the friends. One poor guy was locked up in a ship's brig for three days for having been rude and bigoted at the front counter to the hero's wife. I actually felt bad for the guy. Sure, he was kind of a jerk, but he was also doing his job and tried to protect himself when some stranger woman set a huge dog on him and then put a gun in his face. He didn't deserve being locked away and miserable for that.
Heck, if memory serves me correctly, the hero was only shot it once! Good grief, in most Westerns you can't serve breakfast without someone taking a shot at you. Everything was so heavily stacked against the bad guys that the hero felt more like a mob boss with ties to people in government power, swaying the law in his favor rather than a brave fighter trying to deal some justice. By the end of the book he hadn't even been required to expend much effort and I felt bored. Everything was just too darned easy. Half of the bad guys killed each other off or just didn't have the common courtesy to get shot in the face by the main character.
See what I'm getting at here?
Another difference between typical pulp heroes and Mary Sues is that pulp guys grow up in their craft from childhood, so they've had a lot of time to master it. Tarzan knows how to do tracking, fighting and survival because he was born in the African jungle and was breaking his back from the time he could walk. Conan grew up in the hills of Cimmeria among a tribe of barbarians who's national sport was Warfare, and the harsh climate forged him into a durable warrior. They aren't the most heavily burdened of people in terms of personality, but you understand their history, motivations, and know how they react to a situation. Sues will find something new and begin to master it after a short period of time, or almost instantly. Again, this can actually be done well, but there must be urgency, effort or something else to ad tension. It can't be easy!
As a further example, I just came across a fantastic statement made by the author of Fargo, a darned good Western if I've ever seen one.
"Always, the stronger the villain, the stronger the hero, the stronger the conflict, the stronger the book."
Such strong wisdom in so short a statement! He is of course, absolutely right. If it feels like our hero is far more likely to lose than win, and that defeat lurks around every corner for him, that creates tension and thus excitement. Heck, with this you can actually make the villain likable! Not every character should be the same, but there is one adamant rule that is absolutely inviolable when making a character that is going to show up for any amount of time, and I frequently reject the idea of "Rules" in writing, so pay attention: The character must be interesting.
Of course, since we are humans, even the best written character will be disliked by someone or come across as boring. It's going to happen and there's not much you can do about it. The character must have some sort of intrigue that makes them interesting to look at and follow.
Anyway, back to the thing about Mary Sues, are they really all that evil? I think only if they are boring or annoying. The lines really get blurred in some forms of literature, depending on what the writer is trying to accomplish. But wow things have gotten out of hand with people casting fingers at every character and decrying them as being a Mary Sue and therefore the entire book is crap and must be crucified. For Pete's sake, calm down people. There are way bigger things to worry about in books. If a character is decent and somewhat likable, then what's the problem?
It's also sad that this is often hurled at female authors trying to put out cool characters from their imaginations and have other people latch onto. Honor Harrington comes to mind, as a good friend let me read one of his pieces, and I read the first of the Harrington stories. It wasn't quite my style, but it was darned well written, and Harrington could certainly inspire young girls to be something other than another decoration in Bowzer's castle. You guys know who you are, so stop picking on people trying to inspire others with a pulpy icon.
Bella Swan obviously doesn't count though. Rip her up as much as you want. Bleh.