Alright, maybe it's my inner child speaking, but the movie is nearly perfect as is. The only things I'd change, script-wise, in all of 'Star Wars: A New Hope', involve Greedo and Obi-Wan.
No, no, not the "Han shot first" thing. That's a given. But that scene could stand to make it clearer that Greedo is acting on a private grudge and exceeding the orders Jabba gave him, and that Jabba's still got some mercy left in him. Otherwise Han has no motive to flee the Battle of Yavin--why would he go back to Jabba if he's been explicitly told, "It's too late [to give him the money]." Plus, it makes the scene with Jabba less redundant--if you leave that scene out, Han should just stay the hell away. If you leave it in with both scenes as scripted, you're being told twice in ten minutes that Jabba's pissed, and that Han needs to get back in his good books. The Greedo scene is all about Han's willingness to play dirty if he has to in order to win the fight, nothing more. (Which is why it's even dumber to make Greedo shoot first.)
The other thing I'd change? The fight scene between Obi-Wan and Darth Vader now looks radically out of place next to the elaborate, choreographed sequences of the other movies. Every other film, Jedi are hyper-acrobatic wuxia masters who dazzle us with epic fight sequences...in Episode Four, it's two guys anemically clashing sabers together. Sure, they're both well past their prime, but...it could use work.
Other than that, every frame is perfect.