Sorry to break the groupthink but the real reason the original movies were "great" and the later movies were great is really simple - you saw the first batch when you were 12, and the second batch where you were 35. Nothing could have matched your original 12-year-old's assessment of the movies.
The original movies were amazingly unsophisticated and simplistic *special effects movies*. There had be absolutely nothing like them before, ever. The opening sequence of "A New Hope" sold you on the entire series, it was an absolutely stunning special effects masterpiece and there wasn't a person who saw it who wasn't amazed. At the same time, the acting was dismal, the story was simplistic, formulaic, and utterly predictable, aside from the effects. That doesn't matter to a 12-year old. The other movies, prequels included, were better in a plot/script/acting sense. The prequels followed the same formula with a better plot and acting, aimed at the same kids who were 12 in 1977 and 34 in 1999. Of course, by then, the special effects were not stunning any more, the expectations build up over decades couldn't possibly be met even if it was Citizen Kane in Space.
I have an advantage over most of the rest of you - I was 17-18 when the first movie came out, and possibly more objective. It was a great movie because it had 2001-quality special effects used for a action-packed story. It wasn't great cinema, Lucas was no more skilled in 77 than he way in 99, and the later movies aren't significantly worse.