Mary Sue is no longer simply a name for an author inserting themselves, but a parasite attaching itself to Fan fiction that was once good and holy. Within the past few years, as fan fiction became more and more popular, people have mutilated and bloodied the art.
Fan fiction, for the sub-rock dwellers, is a genre of mostly internet writing that borrows setting, and characters, for an original plot. However, these days, people don't understand the concepts of planning. They simply plop crap onto a word processor and upload it to websites such as www.fanfiction.net, that don't have a filter system.
While Mary Sue's are commonly pretty, intelligent, perfect, and have every guy swooning for them, there is the other side; the Angsty Sue. In order to be more original many authors have turned from making the ideal person, to creating an ideal horrible situation. These Angsty Sue characters usually have broken homes, evil step-relatives, a dead friend or parent, and are usually cutters, suicidal, into 'punk' music, angry and badass.
Angst in fan fiction is ok, and an intelligent original character is also decent. What writers of Mary Sue characters don't know, is realism. Authors aren't discouraged from writing original characters, but sifting through fifty new Mary Sue fan fiction to reach a gem is tedious work.