With Twilight, admittedly not much. But I often need about 3 pages and to see the target audience of a book before I know I hate it. I think of every person who ever recommended Twilight to me, and everyone of them had awful taste in books, had never read a Shakespeare play and thought Dostoyevsky was a made-up name. Some of these people also thought that the second World War was in the '70s. I'm completely guilty of judging things based on target audience and fan base. Admitted, hands-up, guilty.
Harry Potter is good. Well written and enjoyable. More about the finding of oneself and not so much about "An abusive relationship is okay, because having a hot boyfriend is totes important."
Generally, my raging at fanfiction comes from a few friends. I'm anonymously raging at my friends who post awful stories and want feedback that I don't have the heart to give them. Also, it's like every Doctor Who fan goes to the internet and says "Steven Moffat is so terrible, I should be the showrunner." I just want to grab them by the ears and scream: No, no you shouldn't. He's a brilliant writer responsible for the best reviewed episodes of the show's modern era who has been working in TV since before you were born. You would turn Doctor Who into Rose/Ten porn.