Few would deny that 'Superman The Movie' defined the character for a generation (or two). Sure, Superman had been a Boy Scout for quiet a while already, but it was Christopher Reeves' interpretation of the Big Blue that really cemented the character in the public consciousness in a way he never had been before. And so, all over the people, the name 'Superman' was associated with a pure, selfless, Boy-Scoutish hero who was admired and worshipped by all, and who rescued kittens from trees!
And that's the image of Superman that has persisted in pop culture for decades. Much like how, today, the idea of an Adam West-styled Batman punching out villains with a POW! or a ZAP! would be anathema to most Batman fans, the idea that Superman could be a more complex, and somewhat grayer character, who isn't necessarily accepted as mankind's savior at first, tends to make certain sections of the Superman fandom very uncomfortable indeed.
Which is why there is a LOT riding on 'Man of Steel' as far as the future of Superman as a character and a franchise goes. Because its up to MOS to redefine Superman for future generations. The New 52 reboot and the Earth One OGN's can only do so much. MOS needs to be a great enough film that people all over the world feel that Henry Cavill's character IS Superman, the same way they believed Christopher Reeves' character IS Superman 35 years ago! The film needs to successfully convince people that Superman isn't a Boy Scout but rather a powerful albeit sometimes flawed individual who may or may not be the best thing which happened to the world.
If MOS fails, then it will be condemned as an aberration, much like Schumacher's Batman and Robin was an aberration to the Batman franchise, or as a forgettable 'alternate' take on the character and the 'Boy Scout' image of Superman that Reeves and Donner masterfully perfected will persist in public memory. And Superman being a 'Boy Scout' just DOESN'T work in today's world, leaving the Man of Steel biting the dust while the Dark Knight continues to reign supreme.