I write fanfiction myself, but often do agree that most, if not a large percentage of the stuff is rubbish. It's not so much that they're terribly written (though that is a common flaw with them), but that the people who write them seem set on making the character act how THEY want them to be. I can't count how often I've romance fics where the characters have spun off cliche, cheesy gibberish or depicted the awesome badasses as inane prats who act like they do just because no one will *Bleep* them. Another, personal gripe is how some people try to "second-guess" the end of a series, and often pull a poorly conceived ending concept from out of their ass it's often cringe worthy to look at with your eyes. An example is a Legacy of Kain story where Kain went back in history a few times to magically make himself buff and semi restore Raziel's wings. It just seemed like a poorly explain plot device to make the characters more in the ball-park for the author's intended end rather than a plausible conclusion that could happen in the sequel that'll probably never happen (mind you, this story was made before Defiance).
For me, I usually write parodies, which are allow for complete disregard to set standards of the material. Of course, one has to at least still be clever, and not just jam pop-culture reference like it was diarrhea foaming from the author's *bleep*. Like I said, aside from poor writing, another critical aspect to good fanfiction is that it doesn't allieniate the normal fan just to stroke the author's personal desire for Sephiroth to plug Yuna's "Mana fountain".
