AU which really turns out to be retelling the original story with characters switched around.
AU once meant 'how would the story turn out if X happened' but every AU fic I've seen recommended is 'how can we make sure Y happens regardless of the changes we've made to this universe.'
Using Harry Potter fics as an example, at some point there will always be a troll in the dungeon, sabotaged broom incident and a chamber of secrets opened no matter how drastic the changes.
Now I understand that the average 15-18 year old girl who writes fanfiction probably doesn't have the time nor the mastery of the craft to come up with completely new chapters and stories but I wish they would at least try instead of being a slave to canon and repeating what's already written. Once upon a time I saw fanficcers who did try to be original.
When I read an AU I want to read about the consequences of a single change not how everything largely stays the same.
The other thing I hate is AU which don't need to happen for example a University AU if the canon characters are 15-18. Why not just write a 1-3 year future fic when they are likely to be in University?