One thing that can make a Harry Potter fanfic terrible is if it completely goes against the established lore of the series. For example, if it gives Harry a completely different backstory that doesn't fit with what we know from the books. Another aspect could be really bad writing, like having a lot of grammar and spelling mistakes that make it hard to read. Also, if the characters are really out of character. For instance, if Hermione suddenly acts like a silly, air - headed girl instead of the intelligent, strong - willed character she is in the original series.
A 'terrible Harry Potter fanfiction' might be one that has a really contrived plot. Say, suddenly Harry discovers he has a long - lost twin who is the evil mastermind behind everything, and it just seems like a lazy attempt to create drama. And if the author is just using the Harry Potter names and settings to push their own, not - at - all - related - to - the - theme ideas, like turning it into a political rant thinly veiled as a fanfiction. Plus, if it's full of clichés and doesn't bring anything new or interesting to the world of Harry Potter, it can be considered terrible.
It could be because the plot is full of holes. In a good Harry Potter fanfic, the plot should be somewhat consistent with the original series. If this 'raven' fanfic has a really disjointed plot, that would make it terrible.
Well, without having read it, it's hard to say exactly. Maybe it's a fanfic where the author took the concept of Ravenclaw and did something really off - putting with it in the context of the Harry Potter world. It could be that they misinterpreted the traits of Ravenclaw students or made up some really strange storylines related to Ravenclaw that made other fans think it was terrible.
Avoid writing a terrible Harry Potter fanfic by being respectful of the original work. Don't add in elements that are completely out of place, like modern technology that doesn't exist in the wizarding world. Develop your characters well. Give them flaws and growth, not just make them super - powerful from the start. Also, pay attention to the tone. The Harry Potter series has a certain charm and tone, and if you go too far in a different direction, like making it overly comical when it should be more serious or vice versa, it can make the fanfic not so great.
One reason could be poor writing. If the grammar is all over the place and the sentences are hard to follow, it makes the reading experience really bad. Also, if the characters are not well - developed and seem flat, it can ruin the whole story.
Unsettling plot twists can contribute to a fanfic being a 'nightmare' type. For example, if it turns out that Dumbledore was actually an evil mastermind all along and has been manipulating Harry for his own dark purposes. Or if the prophecy was a lie and Harry's whole life has been a cruel joke. These kinds of revelations that shatter our perception of the original story can make a fanfic feel like a nightmare.
A delicate Harry Potter fanfic often has detailed characterizations. For example, it might show the vulnerability of a character like Harry when he misses his parents. It also could be in the way it describes the relationships, like the complex friendship between Harry, Ron and Hermione. Delicate fanfics might use soft language to create an atmosphere, like when describing the warm light of the common room.
A 'sparkling' Harry Potter fanfic often has unique and creative elements. For example, it might introduce new magical creatures or spells that add a sense of wonder. It could also have really vivid descriptions of the magical world, like how the Hogwarts castle sparkles in the moonlight in a way we haven't seen in the original books.
A shittiest Harry Potter fanfic might have a plot that doesn't fit within the magical world's rules. For instance, if it has spells that don't follow the logic of magic in the Potterverse. It could also be that the fanfic is just a copy of other popular stories but with Harry Potter names slapped on. And if the author doesn't understand the themes of the original series, like love, friendship, and the fight against evil, and instead writes something that goes against those themes, it can be considered really bad.
One factor could be a complete disregard for the established lore and characters in the Harry Potter universe. For example, if a fanfic makes Harry a completely different person with no resemblance to his original character traits, like suddenly making him extremely cruel for no good reason. It just doesn't fit with what we know and love about the series.