@Background Pony #B7F7  
http://youtu.be/csNItUI_TT4?t=22s

@Zincy  
More or less. These characters also usually have very little agency in the story because its not about a person responding to shitty stuff happening to them, its all about the shitty stuff happening to them because it will give you such feels!

Looks like someone flunked out of tvtropes.

@gasmaskangel
 
So it’s basically where a Mary Sue meets a Woobie.

@Zincy  
Except stories are usually about getting rid of them.

@SpikeSpiegel  
Loss is indeed a hard thing to handle way. Some amount of it, a certain element of pain and pathos is necessary to create realistic characters, but the loss in and of itself should not define the character unless you’re going for someone who has just been utterly broken by it.
 
A common subtype of Sue is the Misery Sue, a character who exists for tragedy and nothing else. The world and story conspires to fuck them over because its just so sad, and they’re so brave for facing it. Aren’t they special? </sarcasm>
 
Dead parents is a tragic element that can work, but becomes a problem when it is the whole of a character’s motivations or used as an excuse. I call it the Batman Problem.
 
When the death of Bruce Wayne’s parents is used as the impetus for his vigilantism then he works better as a character, but when he never goddamn shuts up about his dead parents and how tormented he is by the trauma he just looks like a mentally ill person.
 
Something I’ve heard is helpful for keeping the plot from being exclusively about the protagonists is to first write out how the story goes if they don’t get involved at all.

@Background Pony #B6D6
 
You just defined every major big-bad ever.

How the hell is an Ursa Major a Mary Sue??? There’s no character and is only on screen for a few seconds!

@gasmaskangel
 
One thing I find good at avoiding people in my stories - especially my novel, but that’s neither here nor there - is, when I, as a writer, begin feeling… “impressed” by a character of my own writing, I cut their page time.
 
Another convention I use is to have the “team” have an amalgamation of skills that allows them to proceed through the story, but drop a hint or two at its dismissive nature - or the pure luck involved in it, or register some loss (The hardest part of character arcs is measuring the amount of “loss” you want for each character).
 
Dead parents are also not a gold mine, no matter what people say. You can put them out of focus, but… well, the smart ones know.

@SpikeSpiegel  
It originates in a parody of Star Trek fanfiction from way back in the day when the way to share fanfiction was in fan run magazines, and it specifically was meant to refer to characters who are perfect, save for maybe one or two “flaws” like clumsiness if the writer has a molecule or two of self awareness and when inserted into the story utterly destroy it.
 
That last part is the key component of a real Mary Sue type character, their presense forces the entire story, all the characters, events and themes, to revolve around them.
 
However as you say it now seems mostly to be used as a slur for any character that someone does not like.

@SpikeSpiegel  
I’m pretty sure he has it right. A Mary Sue completely warps the plot, so that no significant action can be accomplished without her.
 
Even that guy narking about Twilight’s “power-of-plot problem” should be willing to cede the point that there aren’t actually any Mary Sues in FiM as of now.

Are you fucking with me? Sombra’s almost the fucking opposite of Mary Sue.

@SpikeSpiegel
 
Have you seen that “Litmus Test” people like to link? Every character worthy of a major story role ranks at least a borderline Sue on that thing.

@gasmaskangel
 
Got to wonder where people get their “Mary Sue” conventions - it seems like it’s made into a way in such that it translates to “Character I do not like”.

That wanna-be Sauron who got nuked by a shiny rock is nowhere near “sue” potential.

Yeah…. no. No.
 
For starters a Mary Sue needs to completely devour the story they appear in, eclipsing all other issues and characters while drawing them into their orbit.
 
Sombra was barely even given any attention in his own episode till the very end when he was able to actually attack, and has not been mentioned on the show since, as for the Ursa Major, that’s just freaking stupid. Its not even a character, it was a punchline.

@Background Pony #49E9  
(tumbleweed)

You misspelled Princess Twilight Sparkle the overpowered.

A lot of people need to learn what an actual Mary Sue is like.

Why would either of them qualify?