For me, I think it came down to two main elements: the characters and the story. I have no complaints about the visual effects in the prequels; they were good stuff.
Characters: In the OT, we were introduced to the characters before they were put in peril. We saw Luke’s everyday life on Tatooine, and how he longed for something more exciting. We were introduced to Han and immediately knew who he was: the scoundrel and womaniser (with a heart of gold). But it was mostly through Luke’s eyes that we started seeing the greater conflict between Rebellion and Empire. He was the everyman; I could relate to him.
In the PT, the main characters are often Jedi. They’re superheroes. They have amazing powers. They’re cool. But they’re not us. (I know that Luke became a Jedi in the course of the OT, but that just appeals to a lot of people’s hope that they’re ‘special’. But I didn’t find Luke all that sympathetic in RotJ anyway. His problems were suddenly too metaphysical.)
But anyway, in the PT, there never seemed to be time for any real character moments or development. We had to guess at the relationship between Qui-Gon and Obi-Wan, which should have been emphasised to justify Obi-Wan defying the Jedi Council to train Anakin. There didn’t seem to be any respect, let alone friendship, between Anakin and Obi-Wan in Episode 2. I won’t even talk about the Anakin-Padme ‘romance’.
Basically, in the PT, I couldn’t immediately relate to any of the characters because they were all rather esoteric. And the movies didn’t give me any reason to start relating to them or caring about them.
Story: The OT gave us an immediate hook: there’s a civil war in the galaxy. The Rebellion is fighting to overthrow the evil Empire. If our heroes don’t get the Death Star plans to the rebels, the Empire will dominate the galaxy. There was an urgency driving the plot of the first movie.
I didn’t feel any such urgency about Episode 1’s plot. Nasty Japanese aliens are blocking trade routes. Am I meant to care? Is it meant to be thrilling when the characters who haven’t been explored at all win against the nasty Japanese aliens?
ESB had a very simple structure - it was a chase movie. “The Empire is chasing us, so we’d better keep running. Oops, they caught us. Yay, we escaped!”
I have no idea what the storyline of Episode 2 was. Obi-Wan discovers a clone army. People fly to a bunch of different planets. Some evil guy is building an army. The clones come and save everyone. That’s the main failing of the PT, in my opinion - it’s mostly backstory, and that backstory isn’t integrated particularly well into the primary plot of each movie. If the PT had focused more on Anakin’s personal journey - given us more insight into his character, made his romance with Padme more plausible and his friendship with Obi-Wan more convincing… then it could have been amazingly tragic seeing his fall. As it is, he came off like a moody teenager who got suckered into being a Dark Lord of the Sith because he had issues about his mommy.
So that’s what I wanted from new Star Wars movies. Characters I could care about, and an involving story that put those characters in situations that would show us more about who they were. Instead, I got movies filled with a lot of pretty special effects (and admittedly one very kickass lightsabre battle).