Make no bones about it, Star Wars changed how films were made and how we as the public digested them. It affected an entire generation of film makers and cinema-goers. It was the first film that I remember going to see on my own (aged 12). After Star Wars it seemed for a while there that every other film from 1977 onward was a science-fiction film with a large special effects budget. Even James Bond went into space with 1979's Moonraker. Hollywood does love to jump on a 'band-wagon' - the most recent of which is the comic book adaptation.
The Empire Strikes Back, 'stepped-it up' for the entire Star Wars saga though and is still considered by many to be the best of the entire franchise. The film itself, continues to feature highly in many lists of 'The Greatest Films of All Time'. Although, Star Wars itself wasn't ALL light and frothy, Empire definitely came to the party bearing a much darker tone. This one is all about loss and the emotional growth of all of the main characters. Not only do our heroes not win at the end of this one, Han is betrayed by someone he thought to be a friend and then tortured by Vader, 3PO is broken up for scrap and is only just saved from being melted down. Let's face it, our heroes are given a thorough kicking throughout the entire course of this film, only to reach the end where Luke, just barely manages to escape with his life after his duel with Vader (minus a hand), Leia finally admits to her love for Han only to lose him, and Han himself, ends up being bartered away to Jabba the Hutt for his past misdeeds.
Yeah - I don't think I'm spoiling anything for anybody, anywhere by saying: 'No Luke, I am your Father'. Jaw drops to the ground!
Compare the ending of Episode IV to Episode V. The fact that the ending was down and not up, is probably the main reason for it's enduring popularity. This film made sci-fi cool - not just for the nerds, but - for anybody that appreciated a decent plot, good characterisation and plenty of spectacle.
It was the perfect cliffhanger ending, which was what Lucas, with his love of the serialised shorts of the early 30's, fully intended, I think.