The lore, laid out in Alec Guinness' regal timbre, was informed and shaped by the popular entertainment of the 20th century. What Lucas did was express it in succinct, powerful images: Luke gazing at the two moons on Tatooine, Princess Leia pulling back her hood as she slips away from the Imperial forces, and Darth Vader stalking down a hallway. Immediately, the semiotic puzzle clicks together.
Also, as a movie, the thing fucking moves like the Millennium Falcon soaring through hyperspace. After Luke's aunt and uncle are murdered, he leaves his sand home behind for an adventure with an old man, a sexy bad-boy pilot, a pair of bumbling droids, and a furry beast that speaks in grunts. The rescue mission doesn't exactly go as planned, allowing Lucas to wring tension and humor out of the crew's mishaps. (Harrison Ford's delivery of "boring conversation anyway" remains gold standard action movie banter.) The '70s cool of the young actors, especially Fisher as the cutting and quick Leia, gives the sci-fi elements an edge, and the open-ended quality of the final section -- Vader gets away and our heroes get medals -- leaves you wanting more. It's a perfect movie.