Mary Sues and why we hate them.

[Sir Hugsalot 9,348]

Hate is a big word, but I am not a fan of Mary Sue as the concept. 

Every story, even the simplest one is in a way built around character's weaknesses. It's their journey to learn something, better themselves, grow as people. They make mistakes, they face struggles, hone their skills or even learn entirely new skills essential to overcome challenges thrown at them.

Mary Sue completely disables that, because it makes character flawless and immune to any kind of mistakes. They need to pilot a ship? Suddenly they were always capable of that. They need to fight in melee combat? They practiced that in unspecified period of time and somehow that skill hasn't withered with years of no practice. They need to use some foreign language? Oh, they know it too. They don't need to learn anything. They already know all they need, having learned it long before actual story takes place. In even worse cases they didn't even "learn" it at all. They simply can do it. Don't ask questions. They just can.

That effectively removes any tension in the story, You don't feel like there are high stakes, because You safely assume protagonist will beat the challenge anyway. Even in My Little Pony victories against villains, while easy to predict always come from mane 6 learning something new, creating tighter bond, growing. They didn't beat Nightmare Moon until they discovered their bond and what it stands for, for example. They don't just step in and blast villains into oblivion.

Mary Sue is just a trope, that in my opinion works only for completely comedic purposes and has no place in other kinds of stories or at least I never saw it executed well.