Urban Dictionary: Mary Sue
Mary Sue - Wikipedia
I'm not sure what you mean by Mary Sue I've found two writing related definitions. I'm assuming the second one in which a "Mary Sue" is an idealized perfect character.
So.... Actually it is not the character so much, its the ethos of the stories. I am getting more and more disenchanted with the last Honor Harrington books. It is the first four and possibly a few more which really turn my crank. What it mainly is the big picture stuff, and the thinking of the "enemy" which is always logical in some way, which does it. For me the Miles universe just rings less believable and Miles himself rings less believable.