Things in Harry Potter fanfiction that make me avoid certain fics like venereal disease (list in progress):

Draco Malfoy: Head Boy, Slytherin Prince, Veela, Hogwarts bad boy. Anything else I missed?

Hermione Granger: Resident bookworm, Gryffindor Princes, chocolate brown eyes, heartbroken Head Girl. Anything I missed?

Hogwarts shared Head Students' dorm bathroom. Really?!

Character bashing. Really?! Authors feel a need to bash fictional characters? Did said fictional characters kill the author's pet or something?

Dead characters suddenly and inexplicably alive. Cedric, Sirius, Remus, Tonks, Snape, Fred, and Bellatrix (and whole host of other characters) died. Many witnessed them die. For the sake of good story telling let them rest in peace.

Omniscient and omnipotent characters. Worst are the ones that appear out of nowhere to suddenly become the next Voldemort/Dumbledore. Voldermort couldn't get through the wards around the Tonks' house and he had to wait until he had taken over the Ministry so that he could undo the wards that aurors have put around the Burrow. Dumbledore couldn't apparate in and out of Hogwarts (despite the rubbish that the movies would like you to believe.)

Yet in fanfics you find all these super-powerful characters that easily get through wards placed by highly trained wizards/witches and easily over-power same highly trained wizards/witches. Aurors Ron's and Harry's wards in particular are shit compared to the power and skill of these previously unknown super-villains (who most often than not are previously low level lackeys of Voldemort that all of a sudden grew in power after his death.)

Marriage Law. Personally I think this is the worst ever idea to ever come out of fanfiction.

Nobody learned how to ride a bike by reading a book. But you can bet your ass Hermione tried.