Zero Blogs: the Dreaded Mary Sue · 3:03pm Jun 23rd, 2015
As a writer and a brony, I can say I've seen that term flung around A LOT. Though after that whole Alicorn fiasco, I can safely say that not many people know what the term actually means.
There have been several interpretations of the term, but I feel the most complete one comes from former fandom member Bronycurious.
"A Mary Sue is a character so perfect that they are never challenged by the limitations of the story."
This was actually a critique lodged at EQG 1 a lot. With its conflicts being introduced and resolved too quickly, it never really felt like Twilight was struggling at all.
By contrast, Rainbow Rocks had Twilight BARELY hanging on. Her first attempt to stop Adagio only alerted the siren to their magic. And her attempts to write the counterspell were ultimately fruitless. And even when freed, she still couldn't win without a little extra help. It's these struggles that lead to a better character.
But let me clear up a few things before keep going.
Having a protagonist succeed does not make them a Sue.
Contrary to popular belief, people want their heroes to win. Overcoming obstacles is the standard of life. And indeed, having a hero wallow in failure risks losing the audience's investment.
The problem comes with the conflict. Let me use a metaphor here.
Character A is pursuing Character B to get a sacred relic. And character B barricades him/herself behind a steel door. With no equipment or passcode to get the door open, character A tries to break it down with their bare hands. Which scenario is more satisfying to you?
Character A smashes the door open in a single blow and reclaims the relic effortlessly with Character B barely putting up a fight.
Character A batters the door repeatedly leaving large dents in the metal with every blow. After several strikes he/she finally breaks through the metal, wearily outfights on Character B, and slogs his/her way back with the relic.
If you picked scenario 2, then you're well on your way to understanding good conflict.
Scenario 1 would have less of a struggle and therefore more potential accusations of Mary Sue.
So far, I don't feel that any pony character fits the qualification. Not even Alicorn Twilight 
Regardless, sometimes I get questions from people asking how not to make their characters into Sues.
My answers is as follows: Don't think like that. It's too limiting.
Just focus on well-developed characters and good conflict. and you should be fine.
Zero out.
To Twilight:
3430515 You can also consider this my counterpoint to your earlier statement of Twilight being "almost perfect" BTW.
3430556
I never said she was. I just hate the fact that a lot of people seem to portray her as perfect. Thus, I take every opportunity to poke fun at her. (Wait, did I say she was? If I did, then sorry for contradicting myself.)
Turns out, that's only one definition of what a Mary Sue is.
Other definitions include:
1. a character who can make mistakes and never be called out for it (i.e. The Crystal Empire, Twilight's Kingdom, etc.)
2. a character who is seen as the solution to every major problem, even when it makes no sense. (Celestia and Twilight's relationship in a nutshell)