A Mary Sue (Stu, for males out there) is the name for the character archetype which consists of an authors self-insertion, a character with altogether too many unrealistically grouped powers and abilities, both, or some other strange abilities and or quirks that make the suspension of disbelief extremely difficult. In the process of writing, it is highly important to stay away from a Mary Sue character, as such a character is generally boring, not very well developed, and is a protagonist of little interest. They overcome obstacles with few problems, are the sole driving force behind the story, and with few exceptions cannot be expected to do more than cause an otherwise interesting plotline to stagnate.
This is the general idea behind a Mary Sue/Stu and I personally would like to argue that the trend of avoiding a Mary Sue has actually not avoided the problem. A Mary Sues characteristics include exceptional talent, unworldly good looks, and other bits and pieces of perfect