There are plenty are very identifiable reasons to hate the prequels.
-Bad stories.
-Horrid dialogue
-Bad, underwritten, humorless characters
-Jar Jar Binks. Everything about him. He's just an embarrassment in film.
-Overreliance on CGI versus practical effects. This is a huge deterrent because you can in a number of scenes that the actors are walking against a CGI backdrop or talking to something that isn't there.
-A massive conflict in reasoning from Lucas. He says these are kid films and that's why adults don't like the Jar Jar humor, but frames the entire first film around a trade blockade and senate hearings. Greatly conflicting tones.
-Characters exist to tell the audience the plot. Rematch those films, there are countless moments where the characters walk/sit and talk about the plot developments instead of letting them happen.
-Villains with no personality. They're villains because their inherently evil. No reason beyond that.
-**** poor chemistry between Anakin and Padme. The only reason the audience knows they're in love is because their dialogue says they are.
-The second film legitimately does nothing to progress the plot. Other than a small handful of scenes, it adds nothing to the prequel saga
-Bad, atrocious acting. The product of Lucas not knowing how to direct actors and replacing real sets and characters with CGI counterparts