To paraphrase Pirates of the Caribbean, the "shopping list" is more guidelines than actual rules.  Plenty of really good, fleshed-out characters have characteristics of Mary-Sues, but they have things that balance them out.  What makes the sort of Mary-Sue that people spork is when the character has nothing but Sue-ish characteristics. 

I have no problem with moderate Sues in original fiction, as long as they're coupled with decent writing and an interesting story.  However, it is good to know the rules before you go breaking them, so that you know your character's Sue-ish characteristics and can balance them out.

There are always stupid people who flame (give overly harsh reviews to) stories because they have nothing better to do.  Usually, they have nothing worthwhile to say.  Someone who is honestly concerned with the quality of your writing, and not just making you feel bad, will give you a well-rounded critique that points out flaws and strengths and doesn't just say, "OMG, thats a MarySue!"

In fanfic, the big concern about Sues is that they are often coupled with bad writing and/or mangling of canon.  The stereotype of a Suethor in Lord of the Rings, for example, is a Legolas/Aragorn fangirl whose character exists only for living out her creator's romantic fantasies, and has bad grammar to boot.  Because of this stereotype (which is not unfounded--many Suethors are like that), many readers start flaming the minute they see something that might be Sue-ish.  It's unfair, to both authors and readers, but it seems to be an unavoidable part of the fanfic community.