There might be a 'worst' fan fiction where the author doesn't understand the time period of the Harry Potter world. So you have characters using modern technology like cell phones in Hogwarts. Or a story where the relationships between characters are really forced, like forcing a romantic relationship between Dumbledore and McGonagall in a really inappropriate and untrue - to - the - series way.
Sure. There could be fanfictions where the author completely misinterprets the relationships between characters. For instance, making Harry and Draco best friends from the start without any of the rivalry and tension that was built in the original series. Then there are those that have really bad pacing. They might spend too much time on unimportant details in the beginning and then rush through the important parts of the story like the climax. And sometimes, the worst ones have a very unoriginal plot. It's just a rehash of the original books but with some minor, uninteresting changes.
Some fanfictions that are overly long - winded and full of filler content can be considered the worst. They might have pages and pages of a character just thinking about nothing important or going through mundane daily activities that have no real connection to the main plot of the Harry Potter world.
It could be a different ending for a tragic story. For instance, in a 'Romeo and Juliet' first fan fiction, the writer might have made Romeo and Juliet survive and live happily ever after.
Perhaps Harry would have to face Voldemort alone without the support of the character 'you'. So there would be more intense and dangerous duels.
Maybe Harry is unable to perform basic spells because he's so coddled that he hasn't had the chance to practice properly. So, his friends have to constantly protect him from minor threats like a pesky pixie.
Sure. One example could be that Harry is able to understand ancient runes and spells without any formal training. He just looks at them and immediately knows what they mean and how to use them. Another could be that he has an inborn ability to communicate with magical creatures in a way that no one else can, and he uses this to gain their help in battles or to solve problems in the wizarding world.
In this fanfiction, Harry could be a player in the sense of being a political player within the wizarding world. He might be secretly forming alliances with different wizarding families to gain power and influence. He uses his status as 'the boy who lived' to his advantage. In relationships, he could be a bit of a heartbreaker. He goes on dates with Hermione, Ginny, and other girls, but is never fully committed, causing some drama and tension among his friends and the girls themselves.
There could be an immortal wizard who has lived through all the major events in the wizarding world's history and now has a crucial role to play in the fight against a new dark force. Or perhaps an immortal creature that has a special connection to Hogwarts and is guiding the students in a way that no one realizes.
Well, 'Blubber' is also a bad name in fiction. It gives the image of someone who is overweight and perhaps a bit of a joke. It's not a name that would be given to a character with dignity or respect in most cases, and it can make the character seem less important or worthy just because of the name.
One factor could be really bad grammar and spelling. If a fan fiction is full of mistakes, it's hard to read and enjoy. Another might be if it completely goes against the established lore of the Harry Potter universe. For example, if it changes the basic rules of magic without any good reason.