Irvin Kershner directed ESB with Lawrence Kasdan finalising the script which was why it was good, quite a lot of dialogue was changed including the famous "I know" to Leia's "I love you". Originally Lucas had it at "I love you too".
No surprise that the prequels romance dialogue sucked...
The last good movie Lucas directed was 35 years ago (the original Star Wars). The best stuff that has come under his name as a producer n recent years has been due to the efforts of others. Kasdan and Kershner with Empire Strikes Back, Spielberg (director) and Kasdan (screenwriter) for Indiana Jones. Even the "I know" line was ad-libbed by Harrison Ford. When you read some of the ideas Lucas had that were dropped from some of the films LucasFilm produced, you feel glad that someone else was involved. It was Philip Kaufman (later director of The Right Stuff) who came up with the idea of the Ark of the Covenant. It was Spielberg who came up with the boulder chase. Lucas wanted Indiana Jones to be more like a Bond style playboy, until Kasdan and Spielberg reined him in. It was Lucas who came up with the idea for the last Indiana Jones film to involve aliens, crystal skulls and fridges capable of surviving nuclear fallout. Frank Darabont (Shawshank Redemption, Walking Dead) wrote a script, which Spielberg approved of, involving Indiana Jones being pursued by ex-Nazis in South America. Lucas rejected it and decided at one point he wanted to write it himself.
It's amazing how Lucas has managed to fashion himself into some sort of cinematic visionary throughout the years, when in reality his greatest trick was to see that there would be a market for film merchandise. In fact most of the problems people have with him have been the entire focus on control of mostly off-camera matters: the unwillingness to kill of Han Solo (despite Kasdan and Ford believing it should happen) in Return of the Jedi because it might affect merchandise sales, consulting with child psychologists about whether Darth Vader should be confirmed as Luke's father in Return of the Jedi, Jar-Jar Binks, continually modifying his films and refusing to release originals etc etc