As for the editing, yes, Star Wars is magnificent in its flow and pace, as is Empire to a slightly less degree. Let us also not forget credit to Dwayne Dunham for ESB.
The last twenty minutes of ANH are among the tensest in cinema history--even to this day i can still feel the tension as Luke rips down that trench with Darth Vader in pursuit. The breathless finale of Empire works as well, except the ridiculous "alert my star destroyer" sequence that does some major damage to the flow.
I think Marcia was Lucas' biggest asset, she helped bring out the best in his abilities and keep his eccentricities in check while also providing warmth and humanity to the characters.