seanan_mcguire (who is an urban fantasy writer but is also known as Mira Grant and I need to write about her zombie books when my cold abates and I can be more coherent)
posted about Mary Sues and how the term is being misused. I think that I like her definition of the term and the truth is that it has a pretty narrow use but has been expanded in the fandom to the point of loosing all meaning. Mary Sue is any character you don't like but mostly it is any female character (find me one the term wasn't used for). It is also used now for canon characters more often then fan fiction. If this trend preserve soon Maria Skłodowska-Curie and Ada Lovalce will be called Mary Sues too.
It doesn't just mean an author self-insert. This can be bad, really bad as Anita Blake and Richard Rahl prove but it can be fun - there is a Polish writer who uses her own name for the main protagonist of her detective stories and they are pretty good. It also desn't mean that a character has special powers as most of the main characters of any stories (especially genre ones) are special in some ways (or many ways) and that is a plot point not an inherent fault. It can be over done to the point you can start to dislike the character but it has more to do with the bad writing then anything else in a sense that if that happens all the other characters and the plot are bad too. Nothing is more suspicious too me then someone saying this is a great story by a great writer but that Mary Sue ruins everything so make her die.
As I understand Mary Sue is something that stands out form the rest of rest of the story and takes it away from all the other characters when she wasn't suppose to. A character who becomes the story, deux ex machina and centre of the fictional universe. It's funny how rarely this is what get called that name. And as Seanan also notes it is mostly used for female characters that are no more special then male ones but still get accused of being this.
If you have problems with story and characters explain them, give reasons. I may not agree with them but at least I will treat them seriously. Like I feel about, Anita Blake and Sword of Truth books. I think that they are full of badly written characters and poor plots. One devolved into porn the other into preaching with all the characters used as a figureheads existing just for that one goal, especially the main ones. The fact that the main characters are almost invincible and always right is just a small symptom of what is wrong with the books. If you are just using blanket terms such as Mary Sue I will just ignore your opinion as invalid.
What makes it all worse is that not only characters like Starbuck and Daenerys Targaryen get called Mary Sues but also the ones like Cally or Sansa get called boring and useless. Nothing is ever satisfying or good enough or realistic enough. Of course there is plenty of women characters who are just McGuffins or Love Interests or Damsels in Distress (or any combination of the three) that just lack any personality or agency but this is usually bad writing again and tell-sign of a bad book. If you love everything except a female character maybe you should think why before resorting to name calling. Paper cut-out female character is like a number 15 on my list of why Battlefield Earth is a bad book and I usually don't get that far in criticising it.
If you think that author can make all male characters realistic but not female ones so they should be written out do you really think it can still be a good book? Are male ones really that much better? I never though so about the Wheel of Time ones despite fandom insisting on it. I think the main question before calling a female characters any of these names is asking oneself would you do it if the character was male? It surprising how the name calling decreases with gender swap. Unless, of course, he would still interfere with the OTP. I'm always bothered by that kind of misogyny but it's so much worse that it is so strong among female fans. This is why
womenlovefest was such a great idea.
And Seanan linked to a post by Zoe Marriott that really sums it up nicely. I especially love this comment:
Writing really strong, flawed, REAL female characters is tricky not because real women aren't strong and flawed, but because (it seems to me) female MCs just can't *win*. If they're messed up and twisted then they're 'unlikeable'. If they're smart, confident and together then they're dubbed bossy and too perfect or Mary-Sue. If they're gentle and sweet they're called useless and passive. If they're ambitious or into boys they're called bitchy and slutty. You hardly ever see this kind of detailed scrutiny applied to male MCs.
I consider the Mary Sue term so misused and overused that it almost started to reverse itself. I noticed lately that if fandom is foaming about a female character being a total Mary Sue I'm bound to love her, think she is awesome and want her to be even more prominent character. Like an arrow pointing me to what I like.