Hi there~
I´ve been thinking quite a long time about this. Though I should spend my time thinking about school more and less about anime ands stories. That reminds me, I and my friend are now working on our OCs story, though we still deciding how to do it. Nevertheless back to the topic of Mary-Sueness.
If you don´t know what it is, please google it. From my perspective it´s the most common problem with generally most OCs. Even more in Yu Yu Hakusho universe. Personally, I kind of understand it. Oh, come on it is/used to be one of the most viral animes and there are unpaired cannon bishies...  And even if their have their better half it doesn´t stop the fangirls It just asks for it!
BUT ... There´s big but. Sometimes the authors make Mary-Sue out of their characters, not to mention sometimes there is some OOCness of the cannon characters happening.
Now I am going to share my personal opinion. I don´t think it is bad thing to have a bit of Mary-Sue in a character. And it´s not because my OC is super hard-core Mary-Sue!!!   There are even some trait of Mary-Sue/Gary-Stu in some anime cannon characters. Who wouldn´t like to have a characters that people like? Though, everything has its limitation and does OCs. I think when the Mary-Sueish traits are logically and well explained in a way everything makes sense and has some purpose, so whateves just leave it as it is! To judge whether OC is Mary-Sue or not is especially difficult and definetely subjective.
So what I compare her eye-color to a gemstone? It´s the first thing it popped in my mind... Other things are different story though.
Actually, I dislike even more when people purposely make their OC seem non-Mary-Sue. They give her totally opposite personality... Isn´t that Mary-Sue too? Because people are bashing Mary-Sues so much, so they make the total opposite so people would like their OC more. That´s one of the traits of Mary-Sue, isn´t it?
Nevertheless I am not changing my OC just to make her less Mary-Sue. I did change her a bit though, but it was because my own feelings and likings and more importantly because it clashed with the original story.

Okay to the real purpose, what are your thoughts on Mary-Sueness?