Some helpful background on Mary Sue is found on this Making Light post.
And then there's me, below.
I was reading a book by James N. Frey (the writer, not the fraud) - The Key: How to Write Damn Good Fiction Using the Power of Myth. It uses the Joseph Campbell Hero's Journey as a framework, and I found it entertaining even though I'm somewhat dubious about Campbell, the same as I am about Robert Graves.At the time, I didn't go with the answer that's becoming the clearest in recent discussion--that the great difference between a Mythic Hero and a Mary Sue is that the Mary Sue is a female character, especially one written by a female author.
Anyway, Frey provides a handy-dandy checklist of the possible characteristics of the Hero (he neatly end-runs the sexism thing by making his own exemplary Hero a woman):
The Hero is a protagonist, is an 'outlaw', has courage or finds it, is good at his/her trade, has one or more special talents, is motivated by idealism, has been 'wounded', is clever and resourceful, is sexually potent/attractive, is stoic, is loyal, is physically superior, may have a special birth and special destiny.
You'll note I've bolded a few of those. Now, I direct your attention to The Original Fiction Mary Sue Litmus Test, and to the following questions:
Is the character highly attractive? (3 points)
Are one or more other characters attracted to her/him? (1 point)
Is the character related to royalty or nobility? (4 points)
Is the character the last surviving member of a family/clan/race/species/etc.? (2 points)
Was the character ever forcibly banished from her/his family/tribe/country/etc.? (3 points)
Does the character have an angsty childhood, or an angsty past? (1 point)
Was the character abandoned by her/his caregivers? (2 points)
Is the character unusually accomplished for her/his age/species/etc.? (2 points)
Does the character ever easily learn a difficult skill (e.g. learn to play guitar in a matter of weeks)? (3 points)
Does the character have a special birthmark or other marking? (4 points)
Does the character have any particular skill at which she/he the best or among the best? (2 points)
Which made me wonder. Is Mary Sue a mythic hero? An attempt at a mythic hero? Is the Hero a Mary Sue? What's the difference? I'm hesitant to say skill, because the myths have been through so many hands and languages the skill of the telling is kind of hard to be sure of.
How dare a mere gurl act like a mythic hero? What would Joseph Campbell say?
Tangentially, this is one of the reasons why I prefer fairy tales to myths--that the protagonist in the majority of fairy tales is interchangeably male or female. But that's probably another and longer post, for some later time.