DuracellEnergizer said:

Changed the whole method of how Palpatine & Vader go about seducing Luke to the darkside to make it more believeable. The whole "if-you-strike-me-down-in-hatred-you'll-join-me" bit is just dumb and smacks of future PT developments.

This is where Lucas really should've taken a page from Tolkien (whom he obviously read--or else why are Jedi sabers blue and Sith ones red?). The One Ring in the LOTR books, for all its evil power, doesn't turn you into a mindless ally of Sauron. If Denethor, Steward of Minas Tirith, got his hands on the Ring, he wouldn't use it to ally with Sauron and crush the people of Gondor and Rohan--he'd use it to overthrow Sauron utterly and establish Gondor as a mighty empire.

But Tolkien's entire point was that this is the wrong approach. If Gondor defeated the Evil Empire of Mordor using Mordor's own methods, it'd just inevitably set up a new despotism in its place. But Denethor would still think that he was a hero, and still claim to be acting for the forces of Good.

Lucas missed Tolkien's point--that fallen good guys don't stop thinking they're good guys. If Luke kills his father in anger it doesn't make sense that he would become the Emperor's lapdog--it makes far more sense that he would try to overthrow the Emperor and seize power for himself, in the name of creating a new, more "benevolent" Empire. (Mind you, Kasdan actually proposed this as a happy ending for ROTJ.)

FWIW, Peter Jackson got this one wrong too--by turning the traitorous and cunning Saruman of the novels (who happily betrayed Mordor at the first opportunity in hopes of seizing the Ring for his own) into a much more authentically loyal servant of the Dark Lord.

Sorry for the slight off topicness...