There are lots of sites for bashing Self-Insertions (the notorious Mary Sue) out there already. Perhaps, Charisma, you could distinguish your site by taking a more "rehabilitative" angle on the matter. Perhaps one page could give pointers on how to recognize a Mary Sue in your own work or someone else's--complete with illustrative links to both "Mary Sue" and "Not-Mary-Sue" fics. A second page could then give tips and pointers on how to avoid the "Mary Sue" tag.
I say this because I've taken several "creative writing" classes, read a lot of dreck, and blushed after rereading stuff I wrote when I was five (yes, I still have that story, and no, I won't say where I hide it). In my experience, Mary Sues and other Self-Insertion Demons are part of the process of learning to write a good story. "Repeat offenders" of Mary-Sue-dom are simply stuck at a certain level of authorial development--or writers feeling lazy.
A Mary Sue-type character shares some characteristics with the Dread Paraphrasis--they both allow the writer to write without exercising much imagination. A Mary Sue, by its nature, lacks any independent existence--it is simply a projection of the writer's understandable but dull image of perfection. The perfect character tramples all over the laws of probability, human nature, and physics, and is allowed to get away with it by an overly indulgent writer who is just discovering how wonderful it is to have a whole little world at one's whim. One might even argue that the "Mary Sue stage" is necessary to becoming a halfway decent writer--because reading and writing self-insertion stories illustrate some fundamentals of fiction; namely the ones regarding perfection.
I've said it before and I'll say it again--my Torts prof was right--"It's no fun until someone gets hurt." Good fiction always presents the reader with a character who has a problem of some sort or another, whether it be saving the world or learning to tie his shoes. The story follows the character around while he tries to solve his problem. If the story is well-written, the reader soon identifies with the lead character and roots for him as the two of them move through the story. At the end, the character has either solved his problem or failed to, but either way, he has grown and changed, and perhaps his reader has changed a little with him.
To cite from ReBoot: Consider Bob, the hero par excellence . Bob is smart, quick to defend his family and friends, and always has a witty remark ready. What separates Bob from a Mary Sue is that Bob has faults--he's reckless, occasionally sticks his foot in his mouth, mechanically inept (his car is never running for more than a day or two at a time), overly idealistic and insubordinate (witness his determination to rehab Megabyte in the face of Turbo's insistence otherwise) and his love life is prone to random upheaval. To sum it up: Bob's got problems. He makes mistakes and suffers for them--which we can all identify with--so we cheer Bob on when he learns from his mistakes, makes the right choices, and wins the day.
This is one of the points at which Mary Sues fall flat. Mary Sues generally do not have a problem to solve--they're already perfect. If they do have a problem to solve, it's so minor and/or outside the realm of ordinary experience that the reader can't identify anything of herself in the character--and so doesn't really care about the Mary Sue. Without flaws, the character has no room to grow and change, and thus no basis for a story.
Mary Sues are also a common part of the "learning to write" process because they are an intermediate step in developing the imagination. A good fictional character has a mind and life outside that of her creator. Truly wonderful writers can write from almost any perspective--that of an child or adult of either sex (or species, in the case of sci-fi writers), socioeconomic or historical background, in any situation one cares to dream up. (One possible exercise for strengthening the imaginative muscle--play "Freeze and Justify" in your head or with others.) It's harder than it sounds to keep a character separate from oneself--one little narrative voice says "I, the writer, would do this" while another whispers, "But I, the character, would do something else". It's hard to let the character lead the fantasy.
The same imaginative underdevelopment accounts for the bland plots of self-insertion fics. Since the writer overidentifies with her character, she won't allow anything even mildly bad to happen to him, since changing him would require changing her pleasant little fantasy of a static little world where she (through her avatar) reigns supreme. Change in the fantasy means facing change in the real world and in oneself--not an entirely comforting concept. Don't get me wrong, there's a place for unchanging comfort fantasies--but they don't belong in fiction writing intended for public consumption.
Resistance to change restrains the choice of plot elements quite a bit. Torturing other characters happens a lot, because torture can take up a lot of pages without accomplishing much of anything. After all, a character being beaten senseless for no real reason isn't accomplishing much except making the reader wince--any growth or change in how the character reacts to the world occurs after the character escapes the dungeon and can interact with the world and other people again.