Original Characters versus Mary Sues

So I've been working on this a bit, though it's not as coherent as it ought to be yet. But when I was at Anime Boston, and going to all the writing panels, I was really struck by how much people wanted to talk about OCs and Mary Sues. When twigcollins and I briefly touched on it, that's when all the hands went up. And then the next day at another panel, a huge chunk of time was eaten up by discussing OCs. So obviously this is something People Want To Know About.

And so when I was in Japan and stuck in a waiting room before I was allowed to go visit a plant, and after I'd made a list of all the knitting projects I have planned, I started to write up My Thoughts on Yaoi Original Characters.

( How to Handle Original Characters in Fanfic )

So there it is. I'd like to hear your opinions on all of this. I usually figure out my own opinions to things socratically, so it would be really helpful to me too. And I'd like to hear about any good examples of OCs you know of as well. Thanks to twigcollins for inspiring this, and to flidgetjerome for going over it and reminding me of things like villains.