I really think you're underestimating how much of a sense of humor the original film had about itself. I was watching some of it just the other week, and was struck by how much of it was an affectionate comic deconstruction of its own genre. Luke actually said he couldn't see a thing through his Stormtrooper helmet -- that's as meta as anything on [I wrote:
Robot Chicken[/I] today. And the way Luke and Han bumbled through the rescue and then the princess proved to be thoroughly unimpressed was a great way of deflating and lightly mocking the conventions of the adventure genre.
And I should remind you that C3PO and R2D2 had more screen time than practically any other character in the first reel or two of the first movie. The comic relief was always center stage.
Honestly, I'll never understand Star Wars fans who act as though it was meant to be some great serious epic and that treating it with humor is a betrayal or something. It was meant to be a playful pastiche of Saturday matinee adventure serials. The original three films worked because they didn't take themselves too seriously, because they remembered to be fun.
C3PO and R2 's screen time cannot compare to the neverending antics of Jar-Jar, wisecracking battledroids, and the endless aliens (that diner slob pulling his pants up in Attack of the Clones
for one example) and robots played for laughs from the Prequels-forward. The interplay between the droids in the opening of A New Hope
was secondary to the spectable of the overrun Blockade Runner and the appearance of Darth Vader.
On the flip side, by the time the prequels rolled around, Jar-Jar was a constant distraction, and worst of all, he was inserted into the field fight on Naboo as a 100% sight gag. Compare that to 3PO in Return of the Jedi
, where he stays out of the way throughout most
of the fight on Endor. R2--as always--was a functioning protagonist with only minor dips into the humor pool.
Audiences actually rooted for R2.
Jar-Jar was despised from moment one.
Thinking of Jar-Jar, there's a reason the character was near-universally criticized, which explains why Lucas toned him down in episodes 2 & 3: he was over the top, and no one was laughing.