Bitchy Goodness:
Anne W: "Mary Sue" is a plague upon the fanfiction (and even the professional fiction and film world). She is a character that the author inflicts upon the readers. She is beautiful, often in an exotic way, with an eye color rarely found in nature, and hair that should star in a Feria ad. Mary Sue often winds up saving the day, boinking the male lead, and befriending everyone and their dog (or wolf, if she appears in Due South fic). Many times, she winds up being a long-lost relative or bosom friend of one of the main characters. If she dies, it is a heroic sacrifice, and everyone weeps prettily at her funeral. She is often telepathic, frequently misunderstood (but only by characters the author hates), and is charmingly unaware of her own beauty and intelligence.
Mary Sue typically has a beautiful singing voice. Either that, or she plays the flute. If she is a Slayer, she is better than Buffy, if she is a witch, she's often more powerful than Willow. If she's an Immortal, she's older than Methos. She may have been interestingly traumatized sometime in her past. She has no genuine character flaws. The author will generally spend a great deal of time describing Mary Sue's clothing/eyes/hair in an attempt to distract the reader from the fact that MS scores a big zero in the personality department.
Deena: Oh! Mary Sue is me without my sparkling personality!