Kershner is just one bad-ass director. He took the unusable dialogue and had the actors kick it around until it worked.
And when the script was written by Brackett & Kasdan you had them working exclusivly on that aspect of the movie.
For as much as an autuer director that Lucas wants to be, he isn't. He simply lacks the ability to do that. Hell, Speilberg just directs. You don't see him banging out scripts and then making a movie about it.
Making movies is a team process. You have 3 or 4 top dogs who are brilliant in one area but lackluster in others and when you combine them all you get one kick-ass production team. Lucas was too much of a douch bag to realize that when making the PT and apparently skipped out on his peice of the humble pie and just did the whole thing himself. And it sucked.