The typical “character pack” provided with my outline tells me that Mary is “nice, smart,” and other vague adjectives; she rarely gets narrative space to prove it (I'm lucky if I can sneak in a scene of her reading a book). Her qualities are informed ones, emerging not from the story itself but from the lips of Mary's besotted suitors, ever quick to reassure her (and the reader) that she is the single kindest, most beautiful, worthiest girl in the world.
It is this concept of worth I find most troubling. Mary Sue doesn't find love through common interests, through shared experiences, through long conversations or walks along the beach. Her relationships are not predicated on the idea that two people, with all their flaws, might discover themselves operating in emotional synchronicity. Rather, Mary is loved because she is the best (often, the plot demands that she be extra-special, possessing a secret royal lineage, or magical powers exceeding that of even "ordinary" mermaids). That she is the best is proven, somewhat circularly, by the love she inspires: heroes, villains, and minor characters alike must prostrate themselves at her feet: a vicious circle of affirmation.