Essay: What exactly is a Mary-Sue?

What exactly is a Mary-Sue and how can authors turn her into a well-developed original character?

Many of you writers who have developed original characters – both male and female – have probably asked themselves if their character was a Mary-Sue (or the male counter part Gary-Stu), and like I you probably haven't found a sufficient answer. I won't be giving you one as my opinion can't be measure for what the majority of people think, but I'll give you some thoughts on the topic.

My friend narya86 and I are currently working on a Supernatural fanfic, a series of stories to be exact, that will feature two original female characters and love interests for Sam and Dean Winchester. So I've been browsing the web, taking various Mary-Sue litmus tests (e.g. The Universal Mary-Sue Litmus Test which some of you might know) and I've gotten a big variety of results, ranging from "true original and well-developed character" over "borderline MS" to "real Mary-Sue". I don't think our characters are Mary-Sues, although they certainly have some characteristics of her. But I've been pondering this for a while and came to the conclusion that no test could really give a sufficient answer.

( In the following, I will go through some of the questions of the abovementioned test and add my thoughts to it. )

ETA: Sorry if you guys just had the whole thing appear on your flist, but this stupid rich text format editor thingy just drove me insane ;)