When the book Twilight was released, its heroine, Bella Swan was almost universally panned by critics. Many stated along the lines that she lacked any sort of real character and considered her obsessive and flat. In other words, she was dubbed a bonafide "Mary Sue" by just about everyone except perhaps the most die hard fans of the series. For this reason, it's rather easy to pit the blame on this weak characterization of a protagonist as to why Twilight wasn't very good. And why not?
The old logic of writing goes that people will read stories about interesting people doing boring things a lot longer than they will about boring people doing interesting things, so pinning the blame for a story's lack of real development on the Mary Sue character makes sense in theory. But in my opinion, Stephanie Meyer could have thrown in Wonder Woman, complete with Gal Gadot's nuanced interpretation of the character, into her story and it wouldn't fix a much deeper problem with the story: the lack of an actual story structure.
What do I mean by that?
First, it's important to know that when I refer to a "Mary Sue" character, I'm not meaning an unlikable character. Characters can be unlikable and still be strong, nuanced characters. I'm not so much meaning archetypes either. While archetypes are not representations of real people, per se, they can still be interesting and relatable in certain types of stories, particularly morality and cautionary tales. When I say "Mary Sue," I'm meaning more characters that lack any real progress or participation within their own story. By the end of the story, they're same person they were in the beginning. Some will call a Mary Sue character "perfect" and this is not a bad term, because a perfect character has no reason to progress and there's nothing for the reader to learn about the character either. That means nothing to reflect on when the final page is finished.
Think back in grade school, learning the very beginnings of literature and story structure, we're introduced to a chart that details a story's progress. It starts with the exposition, goes into the rising action until it hits the climax. From there, comes the falling action, and then finally the resolution.
When I'm looking at a story involving a Mary Sue character, most times I can pin point where exactly on the chart there is an issue with the story itself and how the author used these parts. Each part of this chart, has to establish something within the story. The exposition of the story is where readers are introduced to two things: a protagonist and a conflict.
Stories like A Brave New World and The Giver have rather large expositions and this is pretty typical for the dystopian subgenre of science fiction. In the meantime, the book Sabriel by Garth Nix has a very short exposition. Long or short, this exposition needs to establish a character's "normal." Even if that normal is horrendous. Within a properly written exposition, three things are established about one or more characters: goals, motives and conflicts. The goal being what the character hopes to achieve, the motive is why they want that and the conflict is the thing that challenges those other things. And the conflict is centered around an antagonist, or something that stands opposite to the protagonist.
I'm not going to go too deep into antagonists and how they work, but the conflict created by the antagonist leads into the events that creates the tension that contributes to the rising action. That tension builds to the climax where the protagonist faces that conflict head on.
And here in lies the problem: Twilight has no real antagonist. Sure, later books in the series do include antagonists, but the first book severely lacks one, at least not a meaningful one. James doesn't show up until well into the end and not the persistent enough threat to contribute to the rising action. And while we're told about Edward's blood lust, is there ever a moment where it poses any real definite threat to Bella? Edward's family accepts her with little conflict. So given these facts, my only conclusion is that there isn't one. And that leads to a climax with nothing to prove. This issue is further complicated by Bella not being an active participant in the climax and resolution of Twilight as she's incapacitated almost immediately.
When a character has nothing to prove, they don't grow as a person, which is explored in the falling action and final resolution. This lack of growth and development is what makes a true Mary Sue character. That's is why a lot of those tests on the internet which asks writers questions like "How many languages does your character speak?" and "Does your character has special powers they don't know about?" fail to assist a writer in creating a more rounded character. These tests only identify certain traits used to explain certain character actions, they don't determine whether the character grows.
Equally as unhelpful is that creating a good, strong character is as simple as adding flaws. This is only partially true. Adding flaws will give a character an extra something to overcome, but if the story itself is broken, there isn't going to be opportunity to overcome anything. Worse yet, too many flaws and it becomes impossible to integrate them all within a character's story arc. At least not in a way that won't feel ham-fisted.
With this all said, there needs to be mention about characters who don't seem to really actively participate in their own stories or are never challenged. This is certainly the case with Bella Swan, but it's also been recently seen in Marvel Comics as well. While the company itself states the issue with the lackluster sales is in an audience not interested in diversity, critics have pointed out Marvel's inability to pit their rebranded heroes with any real challenges and it started to wear on reader's patience. The new brand of heroes aren't as well received as their predecessors because they aren't faced with the same challenges. Without a challenge, the character becomes static and lifeless.
The real truth of the matter is, the only question that a writer should ask themselves is where and how did a character grow? And if any of the flaws make any kind of meaningful contribution to the overall story. And does the conflict challenge the protagonist? These are the things that will truly round out a character and make them much more an interesting read.