1. Han, Luke, Leia
2. Already knowing the story
-The first one was Lucas biggest mountain to climb, as I always thought he had a tough rode to make ANY characters as good as those three from the OT. Even though the PT is different in story, tone, and a tragic ending, you still have to create characters you care for, and I just thought he was not going to strike lightning in a bottle twice.
-The second problem was drama, WE ALREADY KNEW WHAT HAPPENED!!! If Lucas did one Prequel movie soley based on Episode III events, as more of a prologue to the OT, it could have worked cause we wouldn't have waited through 2 setup prequels for the real story. But to watch all these characters on screen in Episode I, and knowing who survives cause we saw Episode IV-VI makes these movies very non-dramatic, and in a sense makes you wait for everything you want to see on screen, instead of the OT story, where you waited to see WHAT happened on screen. I mean do you think a trilogy about Vito Corleone would be interesting? No, Coppola did one movie that sprinkled in flashbacks that gave just enough backround to make it interesting for any Godfather I fan, but not too much where it seemed bloated.
I think prequels are not worth it, because it is just too hard to tell a story that we already kinda know about. Sure there are cool things in the PT, but a 3 movie trilogy is just too much to grasp onto a trilogy that has already been told. I think it is the ultimate irony that the OT is the greatest trilogy in movies, and the PT is the greatest what if trilogy in movies.