It might just be me, but as someone who hangs around review communities (for stimulating discussions) and sporking communities (for shits and giggles), I've found that the term 'Mary Sue' has lost its meaning somewhere along the way. It was a pretty elastic term to begin with what with the plentiful crop of types to choose from
and the lines that blurred between, well, a powerful character and an unrealistically powerful character. But there are Sues and Stus out there and there are several ways
to identify them.
But Mary Sue is a spectrum, not a dichotomy. So in some cases when someone cries "SUE!" someone else might be inclined to say, "I disagree with your assessment, good sir/madam." Some characters have Sue traits. Some good
characters have Sue traits. But does that make them Sues? Well, in my opinion, no, but as subjective a term as Mary Sue is, someone might say that no good character could possess a Sue trait at all. I would say, "Your favorite character was marked at birth as the Chosen One, has the dead parents angst going on, got selected as the youngest Quidditch player in a century, often gets to flout the rules with little to no consequence, routinely saves the day, and is generally well-liked by everyone but the bad guys. Those are all Sue traits and yet Harry Potter is not conventionally considered a Sue." (“You’re calling Harry Potter a Sue!” someone yells from the back row. “Read my paragraph again,” I respond.)
The one thing I've noticed lately is that people tend to lose sight of what a Mary Sue actually is
despite the broad definition. People use Mary Sue when they simply mean a flat character (which a Mary Sue often is, though these things are not synonymous). Or they use it simply when they mean, "I don't like this character and that makes her a Mary Sue, right? RIGHT?" No, it doesn't. Sometimes people don't want to expound on Sueness. They're in a main character position, they get screen time, and I disagree with what they do or how they are portrayed. Isn't that enough? Bad Character = Sue. Original Character in an Already Established Universe = Sue. New Girl At School Who Has a Mysterious Secret and Snags the Heart of Male Character A = Well… probably a Sue.
My other problem with the Mary Sue spectrum is the misogynistic structure it creates. The Mary Sue’s BFF, the Gary Stu, does get face time but nobody goes into a movie or starts a book expecting to meet Gary Stu. But when a medium has a female protagonist it seems going in examining her for Sue traits is the savvy thing to do. More powerful than the men in the series? Must be a Sue. Hypercompetent at what she does? Sue alert. A crush
on the super attractive male protagonist who has some romantic subtext with Male Character B? SUE. (And a whore for good measure.)
The same treatment is rarely—if ever—given to male characters. I once saw a compelling argument (alas, I don't have a link) that stated that if Harry Potter were a female character then the books wouldn't have half of the fanbase that they do now. Why? Because Harry would be labeled a Sue of course.
Another example most of us can relate to: we all agree that Bella Swan is a Mary Sue—every other time I see a Twilight discussion I can always, always
count on "lolol sparkly vampires" and "that Mary Sue bitch Bella Swan." But it's so rare that I see Edward get tacked as a Gary Stu even though he so obviously is. It’s no secret that a good portion of fandom gets its jollies demonizing female characters. (And, unfortunately, it’s a common hallmark of my fellow slash fans.) But the Mary Sue archetype has become so ingrained in the fandom mindset that any strong female or any prominently featured woman or any girl that gets the guy is likely to be scrutinized for their imperfections and then painted over as too perfect, too strong, too magical, too easily forgiven.
It’s lost its meaning. It’s become a catch-all for an unlikable character and, too often, it stands in for critical analysis. Female characters I find generally likeable or well-rounded are getting the shaft for their perceived Sueness. In some parts of fandom, it’s morphed from a legitimate complaint into a witch hunt. I, personally, will not be using it from here on. I know it would be ridiculous to expect everyone to follow suit but I’m adamant in my belief that it’s a term that hurts more than helps. The most I can hope for is that it falls into disuse as the internet and fandom evolve.