Character consistency. If a fanfic keeps the characters' personalities the same as in the movie, it stands out. For example, if Kat is still the strong - willed, feminist she was in the movie. Also, a unique plot that adds to the original story without changing its essence. Like a fanfic that shows what happened during their college days.
The way it captures the mood of the movie. The original '10 Things I Hate About You' had a certain charm, a blend of humor, romance, and teenage angst. A great fanfic will be able to replicate that mood. It might have the same kind of banter between the characters, or the same level of swoon - worthy romance. It should also be well - written in terms of grammar and style so that it's a pleasure to read.
A good '10 Things I Hate About You' fanfic stands out when it explores the relationships in more depth. Not just the main romantic relationship, but also the friendships and family relationships. It could show how Kat's relationship with her sister changes because of her new relationship, or how Patrick's friends react to his falling in love. This extra layer of exploration makes the fanfic more interesting and engaging.