It's impossible to imagine for anyone under 50, but Star Wars was a massive gamble in 1977. Other than 2001, space movies were not particularly successful. And science fiction movies in the 1970s about the future were pretty dystopian: Planet of the Apes, Westworld, Soylent Green, Rollerball, Logan's Run.
A movie with a bunch of then unknown actors -- if you don't count Harrison Ford's supporting role in American Graffiti -- that went $3 million over budget while shooting dragged on was really not expected to do much at the box office. In fact, it opened as a limited release.
But what a movie it was. I saw it the second weekend it was released nationwide, then took my father the next night because there were still things I hadn't caught during the first viewing. The special effects were groundbreaking for the time. The story stole heavily from Tolkien and Kurosawa -- and Lucas still can't write dialogue worth a damn -- but the enthusiasm of the actors and a fresh approach to a cliffhanger style action film grabbed us all. Ralph McQuarrie's concepts and paintings really made the whole thing believable, which is why sci-fi struggles sometimes with a broad audience.
EDIT: The John Williams soundtrack absolutely makes the movie. Each character has a motif and the interplay of those themes as the story progresses, building to the final fight and eventual medal ceremony is amazing.
But James Earl Jones made Vader way more menacing than just the physical presence of David Prowse. You really felt the Dark Side in his delivery of each line. When the movie ended, you felt like you had taken a two-hour rollercoaster ride. The applause and cheering at the end credits was real and deserved.
I still remember debating whether Vader was truly Luke's father after Empire Strikes Back or if he just said it as a diversion, because the idea was almost too far-fetched to take seriously, based on the original. I get that the entire saga is about Anakin, not Luke. But it really took away the premise of a real fight to the death over the Force. Yeah, Vader dies in the end but it's almost a pulled punch by then.
And what a complete bummer to find out 20 years later Vader was only a smirking Hayden Christensen who went rogue.