I don't mind a good mary-sue from time to time as long as she isn't too perfect. Yes she can be beautiful, intelligent, and powerful, but she needs a flaw and most writers fail to give her that one thing that would make her human. Flaws are what makes humans perfect. Dumbledore, as powerful and intellegent as he is, makes mistakes, Harry is still growing and learning, Hermione is wrong from time to time. That's what we love about these charectors. They are real to a point. They can preform magic yet still be like kids we went to school with, like teachers we had in school.

What I'm saying is that no one is 100% perfect, there will always be something wrong with an individual. That is the downside of most Mary-Sues, they are too perfect. They got the looks, the power, the smarts, and they step in the way and can never be wrong. I've ran into too many of those fics and it just makes my stomach wrench and want to rip my eyes out.