The Clones Keep Droning: You'll not get argument from me (or from most people) that Lucas is woefully inept at crafting dialogue, suspense and drama in his scripts. In fact, it's quite clear that were his scripts submitted today, by any other name, they would likely be sent back in a plain envelope, barely read.
But, what made the first three so great is that they were not only ground-breaking, they created a new genre of film - one in which the special effects not only didn't get in the way of the movie, they greatly enhanced it. Everything before this one was shlock (harken back to "The Incredible Shrinking Man" or even "War of the Worlds"). Those movies used special effects because the plot required them, not because they were particularly effective.
Lucas changed all that with his obsession with technology and his unique abilities to milk from that technology every last drop of awe.
Today, not to sound older than my 33 years, the kids's pulses barely quicken seeing all the pod racing, light-sabre dueling, and spaceship hurtling. Interestingly, most of the younger audience grew up on Star Wars as a videogame, not a movie. Sure, many if not all have seen the old movies, but dollars-to-donuts, those first three movies were not their first exposure to the saga.
Last, and where I will differ with you somewhat directly, is that the first three movies had some quality actors; there is no disputing that Harrison Ford's scenes were always the most entertaining, and that Carrie Fisher brought some believability to a female heroine who could occasionaly rescue herself, not to mention her cohorts. Sure, Mark Hamill was no great thespian, and the dialogue was often wooden, but it was all delivered with such joy and enthusiasm - the actors knew they were onto something new, and their own pleasure could not be hidden.
Today, while we get some good actors (Sam Jackson, Natalie, Ewan), they have no joy, no sense of being enthralled by the product. And of course, the dialogue, wooden pacing, and Jar Jar cannot help that predicament.
Still, while Star Wars is designed for kids, I'd argue the first three (or at least the first two) were plenty for adults to dig. Once merchandise tie-ins became de riguer, well, we got what we asked for.