Summary: guys “A Trekkie’s Tale” is four paragraphs long what do u want from me :v
So, history lesson: Once upon a time, before everything changed when the Internet showed up, fanfiction was in its infancy and thus a more limited resource than it is now. Since ninety percent of anything is trash, this meant the ten percent that wasn’t was an even more limited resource. This was frustrating. More frustrating was that a lot of the trash was the same type of trash (which is still a thing, by the way, not saying it’s not, but there are better ways of picking it out now, at least in fanfic; professional fic’s a whole ‘nother story), and even though most everyone knew it was trash people still kept writing it and reading it, taking up pages and brain space that could have been given to something halfway decent.
Then in the early 70s, Paula Smith, an extremely active member of several fandoms, published a flash fiction piece that condensed all of the problems of a common Star Trek sub-type into a single story.
Mary Sue in her original incarnation is an example of what’s now called a Purity Sue—basically, she’s too good for this sordid world and everyone except her knows it. She would have faded into obscurity, except, again, Paula Smith was not an obscure name, and she and her circle of friends began to use the term to refer to stories and characters of a similar type. With the Internet, it gradually spread out of the Star Trek fandom and even out of fandoms entirely.
And then the controversy starts. (The better question is when does it end.)
A Mary Sue is a character who’s too perfect. How do you judge that? At what point does a competent character become too competent? Can a character with all the trademarks of a Sue not be one if the writer presents them well enough? What even are the trademarks of a Sue? Can only female characters be Sues, and if not, what are the qualities that mark a male Sue? Can a character who everyone in the audience loves still be a Sue?
There are characters who are generally accepted to be Sues (Bella Swan and Edward Cullen were the most acceptable targets I knew of up until their pallet-swapped versions made it to print) and it doesn’t stir up much argument when they’re referred to as such. But when the term is used not infrequently simply to refer to characters someone dislikes and the definition itself is so subjective, it’s hard to tell the difference between someone making an actual argument and someone using a word they don’t know the meaning of as a generic insult. The knee-trigger response for a lot of people by this point is to just assume the latter.
The term itself doesn’t show up quite as frequently anymore, but a concept that was nameless before has been labeled and identified. People know it exists now, and people know to avoid it. Depending on who you ask, that might not be a good thing either.