This posting was inspired by that hilarious Mary-Sue thread. If you're a fic writer--or any kind of writer, really--you need to spend some time over at www.tvtropes.org
Despite the name, it covers everything.
There is a TON of stuff about fanfic, Mary Sues, etc., including an excellent and comprehensive description of the term "Mary Sue" itself. (Although the fic entries don't usually cover Beatles fic, our heroes have a decent amount of information on the site, being Trope Namers.)
Be warned--this site is a time sucker! But if you write, you ought to find it incredibly helpful (not to mention fascinating). For example, keeping track of the tropes you use in your own writing is a very useful exercise. At least, I thought it was! ;)