Back then, sequels weren't formulaic, so to see the heroes losing, especially after such a triumphic victory in Star Wars, was far out.
Then, towards the end of the film, was this big reveal. I was only 6 years old, but I clearly remember the audience gasping, and there was silence - in a Star Wars sequel. Amazing.
I remember thinking, "Vader is the bad guy, he's a liar and just trying to trick Luke into following him." Made sense at the time.
See, back then we didn't read too much into the film(s). They were simple, enjoyable stories, not dissected in minutae on the internet...I seriously believe that if the Prequels came out before the age of mass communication, they'd be a lot more enjoyable. People's perceptions tend to influence our own - so much so that our opinion becomes that of the masses.
It was a simpler time, and man oh man was it fun...