Mary Sue is probably the second most-hated creature spawned by the Internet, just behind JoshCube. And that bitch has spawned more badfics than JoshCube's obsession with Marin.
Who is Mary Sue?
You remember that really annoying girl you were friends with, but not really?
She always talked about the stuff she had or was going to buy; her clothes or someone else's clothes; her boyfriend or all the guys that liked her; and the unending drama in her life.
Remember her?
Remember how annoying she was? How you could never get a word in edgewise, even when your hamster died, and you were devasted? And how no matter how many clues you dropped, she didn't understand that you secretly hated her guts?
The loathing you felt for that annoying, craptacular bitch is the same loathing we feel toward your Mary Sue, because that bitch is your Mary Sue.
If you're trying to convince yourself your main character (and trust us, she's the main character of the fic, why would you do something sensible, like have one of the canon characters as the main character?) isn't a Mary Sue, quit trying to fool yourself.
Why did you wish that once-apon-a-time bitch's organs would sponteously liquidify? Because it was all about her. Plus, she wouldn't shut up.
So why won't you shut the fuck up about how wonderful your avatar (and by extension, you) is?

Wow
Man I wish y'all had been around a few years ago when the plague of Mary-Sues was a downright epidemic.