In some ways, making George Lucas' dialogue sound cool and natural is just as much a challenge as making Shakespeare accessible to modern audiences, which is probably why Sir Alec Guinness knocked it out of the park.
I remember reading somewhere that's a reason for Hayden Christensen and Natalie Portman's terrible performances in the prequels. They didn't know how to deliver those lines, while the old pros like McGregor, McDiarmid and Jackson know when it's time to chew the scenery.
I'm also of the opinion that one of the reasons the Han Solo/Princess Leia romance in EMPIRE actually has some snap to it is because the same writer who helped adapt THE BIG SLEEP also worked on ESB's script.