Here’s a writerly heresy for you!
There’s nothing wrong with ‘Mary Sues’ that a couple of character flaws won’t fix.
The original problem* with the Mary Sue character was that she (or occasionally he) was too incredibly perfect. They got to be beautiful, brilliant, have the hottest lover(s) evah, and between bouts of heroism, had the most unbelievably perfect life.
Which is very boring to read about.
In reaction to this negative label, everyone very sincerely swore off writing too-perfect characters. We (might as well include myself here) would ask ourselves, is this a Mary Sue? What should I do to keep my MC more natural? Chastened writers began to write about much more ordinary and struggling characters. Not a bad thing, since everyone prefers characters we can relate to, but the problem then became the fact that the characters were, by their increasingly mundane natures, unheroic. If your main character saves the day by accident, what exactly is driving the story? Constant internal waffling? Repetitive failures that somehow lead to an eventual win?
I’m so tired of goofy/klutzy/indecisive/low self-esteem main characters who can only just barely manage to save the day via some loophole in the story, just because the writer was too afraid her heroine might otherwise be dismissed as a Mary Sue.
Plus, we’re back to the fact that it’s all very boring to read about.
Fortunately, I’m seeing a return of heroines, who are akin to Mary Sues, but toned down a couple of notches to make them relatable. Heros should be slightly larger than life, should be capable of making the move to fix things, and having the power to back up their actions.
*There’s other stuff that went on with this too, but it’s more political-sociological, so I’m just going to leave it out for this post.