Hmm... I think that the OT is basically the cinematic equivalent of Beatles music. The Beatles were tuneful, with some really catchy hooks, and they came along at just the right time. Paul McCartney is a good singer but far from a great one, Ringo wasn't that hot as a drummer and don't even get me started on what "I am the eggman, they are the eggmen, I am the Walrus, goo goo ca choo" is all about. But somehow the band as a whole is greater than the sum of its parts.
The OT has some clunky dialogue and the odd plot hole, but it came along at just the right time and gave the world's moviegoers the kind of fun experience that they'd been missing for a long time. I think that's a large part of the reason why they were so successful.
If they were to be made today, I think they would still be successful, but perhaps not quite as much so. Take Pirates of the Carribean: structurally very similar to the original SW movie - the hero's journey, rescue the beautiful damsel from the evil villains, character archetypes that fulfil similar functions in the story: Will Turner - Luke Skywalker, Elizabeth Swann - Princess Leia, Captain Jack Sparrow - Han Solo/Obi-Wan Kenobi, Bootstrap Bill - Anakin Skywalker, Captain Barbossa - Vader/Tarkin. It's the classic mythological fairytale that gets repackaged every few years for a new generation.
The fact that this movie and its sequels were very successful says something about the story's timeless nature, but the OT was phenomenally successful because of the timing of its release, and the fact that the audience hadn't seen anything like it before.
Now, if George Lucas had decided to start in 1977 with Episode I, then I think the Episode I that he'd have made would have been very different from the version that eventually came out in 1999. Lucas was a young filmmaker whose greatest success at the time was American Graffiti - he didn't have any of the clout that he had later (and which he arguably doesn't even need now due to his almost complete autonomy as a director/producer/studio), and there's no way that 20th Century Fox would've let him make TPM the way it was eventually written for the reasons that Starship Polaris has already stated nor, probably, would he have wanted to. As a much younger man and seemingly much more in touch with the zeitgeist he would've likely churned out something that was much more accessible on its own merits.