The prequels lacked the one thing the original movies had a ton of: a great sense of tension and danger for the protagonists.
Original Star Wars was about a small group of people fighting incredible odds against an overwhelming power and coming out on top. It was a roller-coaster of danger from beginning to end rooting for the little guy as they barely scraped by.
The prequels were about a lot of people who were too blind and stupid to see a tiny threat become a huge problem. Not a lot of tension in that especially since we in the audience already know the exact nature of the problem, the outcome, and have to watch the protagonists fumble around in the dark for three movies until its too late. The prequels were essentially the Keystone Cops in Space. With the Jedi being the bumblers. Not very intriguing to the audience.
Another problem is the lack of likable characters. It was always going to be tough to make new movies showing a likable version of the baddest bad guy in the universe, but they didn't even attempt it. Anakin was always a jerk and only got worse. The Jedi were out of touch. The politicians were monotone bores. Obi-Wan couldn't save it.
The sequels need to bring back the tension of the originals, but I don't exactly know how they'll do that given that the antagonists were soundly defeated in Return of the Jedi. Hope they don't try a repeat of the prequels with a lurking danger.
Can senior versions of Luke, Han, and Leia be enough of a return of likable characters to prop up the movie? They'll certainly help, but we could just be seeing a repeat of Indiana Jones and the Kingdom of the Crystal Skull.
In any case these new movies can't be any worse than the prequels. That was rock bottom.