American Hominid said:
... the original film has a sense of adventure and "haphazardness," ...
I've been meaning to weigh in on this discussion because it is the very reason why I'm a one-film-only fan. I'll not bore the board - yet again - with the minutia of it all. I will, however, point out that the essence of your statement is what spoke to me when I sat in the theater in 1977.
I think that Empire and Jedi, while they pile on the additional story mythology and relationships, do not fully shift the feel.
They may not fully shift the feel, but they very much get the ball rolling. Empire starts the shift, Jedi completes it. Can't speak to the tone of the prequels because I barely remember the only one I've seen.
With a great deal of input from Marcia, Kurtz, and McQuarrie, Lucas got lucky in 1977. A stand-alone film that works because it's stand-alone. As soon as Lucas started trying to write more story, he ruined the story. The "adventure and "haphazardness," disappeared when Lucas shrunk the universe. Particularly the haphazardness of it all.
Where you see a shift in tone as the franchise goes along, I see it just the opposite. To me, Star Wars is the only film in the series that didn't have that tone. As I've stated before, I think Star Wars is the odd film out.
Personally, I think the original Star Wars trailer is the best example of how different that film is from the rest of the franchise. It's truly an adventure in a galaxy far far away, with a darker tone. Actual darkness - not the cartoon darkness of the other 5 films.